Mode switching by a ventricular leadless pacing device

ABSTRACT

In some examples, a leadless pacing device (hereinafter, “LPD”) is configured for implantation in a ventricle of a heart of a patient, and is configured to switch between an atrio-ventricular synchronous pacing mode and an asynchronous ventricular pacing mode in response to detection of one or more sensing events, which may be, for example, undersensing events. In some examples, an LPD is configured to switch from a sensing without pacing mode to an atrio-ventricular synchronous pacing mode in response to determining, for a threshold number of cardiac cycles, a ventricular depolarization was not detected within a ventricular event detection window that begins at an atrial activation event.

TECHNICAL FIELD

The disclosure relates to cardiac pacing, and more particularly, tocardiac pacing using a leadless pacing device.

BACKGROUND

An implantable pacemaker may deliver pacing pulses to a patient's heartand monitor conditions of the patient's heart. In some examples, theimplantable pacemaker comprises a pulse generator and one or moreelectrical leads. The pulse generator may, for example, be implanted ina small pocket in the patient's chest. The electrical leads may becoupled to the pulse generator, which may contain circuitry thatgenerates pacing pulses and/or senses cardiac electrical activity. Theelectrical leads may extend from the pulse generator to a target site(e.g., an atrium and/or a ventricle) such that electrodes at the distalends of the electrical leads are positioned at a target site. The pulsegenerator may provide electrical stimulation to the target site and/ormonitor cardiac electrical activity at the target site via theelectrodes.

A leadless pacing device has also been proposed for sensing electricalactivity and/or delivering therapeutic electrical signals to the heart.The leadless pacing device may include one or more electrodes on itsouter housing to deliver therapeutic electrical signals and/or senseintrinsic depolarizations of the heart. The leadless pacing device maybe positioned within or outside of the heart and, in some examples, maybe anchored to a wall of the heart via a fixation mechanism.

SUMMARY

The disclosure describes a leadless pacing device (hereinafter, “LPD”)that is configured for implantation in a ventricle of a heart of apatient, and is configured to switch from a sensing without pacing modeto an atrio-ventricular synchronous pacing mode in response to detectionof one or more ventricular undersensing events. In some examples, theLPD is also configured to switch between an atrio-ventricularsynchronous pacing mode and an asynchronous ventricular pacing mode inresponse to detection of one or more atrial undersensing events. In anatrio-ventricular synchronous pacing mode, the LPD times the delivery ofa pacing pulse to a ventricle of a heart of a patient relative to anatrial activation event, which may be an event that leads to acontraction of an atrium. In the asynchronous ventricular pacing mode,the LPD is configured to deliver a ventricular pacing pulse if it doesnot detect an intrinsic ventricular depolarization within a VV intervalthat begins when a previous intrinsic ventricular depolarization wasdetected, or when a previous ventricular pacing pulse was delivered.

In some examples, while the LPD is in an atrio-ventricular synchronouspacing mode, a processing module of a therapy system including the LPDmay determine, e.g., based on an electrical cardiac signal, whether anatrial activation event of the heart is detected within an atrialactivation event detection window that begins at a ventricularactivation event. In response to determining the atrial activation eventis detected within the atrial activation event detection window and inresponse to further determining a ventricular activation event was notdetected subsequent to the detected atrial activation event (e.g.,within an atrioventricular (AV) interval beginning when the atrialactivation event was detected), the processing module may control theLPD to deliver a ventricular pacing pulse according to theatrio-ventricular synchronous pacing mode. However, if the processingmodule does not detect an atrial activation event within the atrialactivation event detection window, and determines that an atrialactivation event was not detected within atrial activation eventdetection windows for a threshold number of cardiac cycles, then theprocessing module may detect an undersensing event. In response todetecting the undersensing event, the processing module may control theLPD to switch from the atrio-ventricular synchronous pacing mode to anasynchronous ventricular pacing mode.

In one aspect, the disclosure is directed to a method comprisingreceiving, by a processing module, an electrical cardiac signal of apatient sensed by a leadless pacing device while the leadless pacingdevice is in a sensing without pacing mode; detecting, by the processingmodule and based on the electrical cardiac signal, an atrial activationevent; determining, by the processing module and based on the electricalcardiac signal, a ventricular sense event was not detected within aventricular event detection window that begins at the atrial activationevent; and controlling, by the processing module, the leadless pacingdevice to switch from the sensing without pacing mode to anatrio-ventricular synchronous pacing mode based on the determinationthat the ventricular sense event was not detected within the ventricularevent detection window, wherein controlling the leadless pacing deviceto switch to the atrio-ventricular synchronous pacing mode comprisescontrolling the leadless pacing device to deliver a pacing pulse to thepatient according to the atrio-ventricular synchronous pacing mode.

In another aspect, the disclosure is directed to a leadless pacingsystem comprising a leadless pacing device configured to sense anelectric cardiac signal and configured to operate in a sensing withoutpacing mode and an atrio-ventricular pacing mode, and a processingmodule configured to receive the electrical cardiac signal sensed by aleadless pacing device while the leadless pacing device is in thesensing without pacing mode, detect, based on the electrical cardiacsignal, an atrial activation event, determine, based on the electricalcardiac signal, a ventricular sense event was not detected within aventricular event detection window that begins at the atrial activationevent, and control the leadless pacing device to switch from the sensingwithout pacing mode to an atrio-ventricular synchronous pacing modebased on the determination that the ventricular sense event was notdetected within the ventricular event detection window, wherein theprocessing module is configured to control the leadless pacing device toswitch to the atrio-ventricular synchronous pacing mode by at leastcontrolling the leadless pacing device to deliver a pacing pulse to thepatient according to the atrio-ventricular synchronous pacing mode.

In another aspect, the disclosure is directed to a system comprisingmeans for detecting, based on the electrical cardiac signal sensed by aleadless pacing device while the leadless pacing device is in a sensingwithout pacing mode, an atrial activation event; means for determining,based on the electrical cardiac signal, a ventricular sense event wasnot detected within a ventricular event detection window that begins atthe atrial activation event; and means for controlling the leadlesspacing device to switch from the sensing without pacing mode to anatrio-ventricular synchronous pacing mode based on the determinationthat the ventricular sense event was not detected within the ventricularevent detection window by at least controlling the leadless pacingdevice to deliver a pacing pulse to the patient according to theatrio-ventricular synchronous pacing mode.

In another aspect, the disclosure is directed to a computer-readablestorage medium comprising instructions that, when executed by aprocessing module, cause the processing module to: detect, based on anelectrical cardiac signal sensed by a leadless pacing device while theleadless pacing device is in a sensing without pacing mode, an atrialactivation event, an atrial activation event; determine, based on theelectrical cardiac signal, a ventricular sense event was not detectedwithin a ventricular event detection window that begins at the atrialactivation event; and control the leadless pacing device to switch fromthe sensing without pacing mode to an atrio-ventricular synchronouspacing mode based on the determination that the ventricular sense eventwas not detected within the ventricular event detection window, whereincontrolling the leadless pacing device to switch to theatrio-ventricular synchronous pacing mode comprises controlling theleadless pacing device to deliver a pacing pulse to the patientaccording to the atrio-ventricular synchronous pacing mode.

In another aspect, the disclosure is directed to a method comprisingreceiving, by a processing module, an electrical cardiac signal sensedby a leadless pacing device while the leadless pacing device is in anatrio-ventricular synchronous pacing mode; detecting, by a processingmodule, a first atrial activation event; determining, by a processingmodule and based on the electrical cardiac signal, a second atrialactivation event was not detected within a detection window that beginsat the first atrial activation event; and controlling, by the processingmodule, the leadless pacing device to deliver pacing pulses to aventricle of a patient according to an asynchronous ventricular pacingmode based on the determination that the second atrial activation eventwas not detected within the detection window.

In another aspect, the disclosure is directed to a leadless pacingsystem comprising

a leadless pacing device configured to sense an electric cardiac signaland configured to deliver pacing therapy to a heart of a patient; and aprocessing module configured to detect, based on the electric cardiacsignal and while the leadless pacing device is in an atrio-ventricularsynchronous pacing mode, a first atrial activation event, determine,based on the electric cardiac signal, a second atrial activation eventwas not detected within a detection window that begins at the firstatrial activation event, and control the leadless pacing device todeliver pacing pulses to a ventricle of a patient according to anasynchronous ventricular pacing mode based on the determination that thesecond atrial activation event was not detected within the detectionwindow.

In another aspect, the disclosure is directed to a system comprisingmeans for detecting a first atrial activation event based on anelectrical cardiac signal sensed by a leadless pacing device while theleadless pacing device is in an atrio-ventricular synchronous pacingmode; means for determining, based on the electrical cardiac signal, asecond atrial activation event was not detected within a detectionwindow that begins at the first atrial activation event; and means forcontrolling the leadless pacing device to deliver pacing pulses to aventricle of a patient according to an asynchronous ventricular pacingmode based on the determination that the second atrial activation eventwas not detected within the detection window.

In another aspect, the disclosure is directed to a computer-readablestorage medium comprising instructions that, when executed by aprocessing module, cause the processing module to: detect, based on anelectrical cardiac signal sensed by a leadless pacing device while theleadless pacing device is in an atrio-ventricular synchronous pacingmode, a first atrial activation event; determine, based on theelectrical cardiac signal, a second atrial activation event was notdetected within a detection window that begins at the first atrialactivation event; and control the leadless pacing device to deliverpacing pulses to a ventricle of a patient according to an asynchronousventricular pacing mode based on the determination that the secondatrial activation event was not detected within the detection window.

In another aspect, the disclosure is directed to a computer-readablestorage medium comprising computer-readable instructions for executionby a processor. The instructions cause a programmable processor toperform any whole or part of the techniques described herein. Theinstructions may be, for example, software instructions, such as thoseused to define a software or computer program. The computer-readablemedium may be a computer-readable storage medium such as a storagedevice (e.g., a disk drive, or an optical drive), memory (e.g., a Flashmemory, read only memory (ROM), or random access memory (RAM)) or anyother type of volatile or non-volatile memory that stores instructions(e.g., in the form of a computer program or other executable) to cause aprogrammable processor to perform the techniques described herein. Insome examples, the computer-readable medium is an article of manufactureand is non-transitory.

The details of one or more examples are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual diagram illustrating an example leadless pacingdevice configured to deliver atrio-ventricular synchronous pacing andasynchronous ventricular pacing.

FIG. 2 is a conceptual diagram illustrating a leadless pacing systemthat comprises the example leadless pacing device of FIG. 1.

FIG. 3A is a conceptual diagram illustrating another example leadlesspacing system that comprises a leadless pacing device configured todeliver atrio-ventricular synchronous pacing and asynchronousventricular pacing.

FIG. 3B is a conceptual diagram illustrating the leadless pacing deviceof FIG. 3A.

FIG. 4 is a functional block diagram illustrating an exampleconfiguration of the example leadless pacing device of FIG. 1.

FIG. 5 is a timing diagram illustrating normal conduction timing in apatient.

FIG. 6A is a timing diagram illustrating an example technique forcontrolling a leadless pacing device to switch from a sensing withoutpacing mode to an atrio-ventricular synchronous pacing mode.

FIG. 6B is a timing diagram illustrating another example technique forcontrolling a leadless pacing device to switch from a sensing withoutpacing mode to an atrio-ventricular synchronous pacing mode.

FIG. 7 is a flow diagram of an example technique for controlling aleadless pacing device implanted in a ventricle of a heart to switchfrom a sensing without pacing mode to an atrio-ventricular synchronouspacing mode.

FIG. 8 is a flow diagram of an example technique for delivering pacingpulses to a ventricle of a heart in accordance with an atrio-ventricularsynchronous pacing mode.

FIG. 9 is a timing diagram illustrating an example of asynchronousventricular pacing.

FIG. 10 is a flow diagram of an example technique for delivering pacingpulses to a ventricle of a heart in accordance with an asynchronousventricular pacing mode.

FIG. 11 is a flow diagram of an example technique for controlling aleadless pacing device implanted in a ventricle of a heart to switchfrom an atrio-ventricular synchronous pacing mode to asynchronousventricular pacing mode.

FIG. 12 is a timing diagram illustrating example application of thetechnique of FIG. 11.

DETAILED DESCRIPTION

In some cases, a dual-chamber implantable pacemaker is implanted withina pocket within a patient's chest, and coupled to a right-atrial leadand a right-ventricular lead. The right-atrial lead extends from theimplantable pacemaker in the pocket to the right atrium of the patient'sheart, and positions one or more electrodes within the right atrium. Theright-ventricular lead extends from the implantable pacemaker in thepocket to the right ventricle of the patient's heart, and positions oneor more electrodes within the right ventricle.

Such dual-chamber implantable pacemakers sense respective cardiacelectrical activity, e.g., respective cardiac electrograms, via the oneor more electrodes implanted within the right atrium and the one or moreelectrodes implanted within the right ventricle. In particular, suchdual-chamber implantable pacemakers detect intrinsic atrialdepolarizations via the one or more electrodes implanted within theright atrium, and intrinsic ventricular depolarizations via the one ormore electrodes implanted within the right ventricle. The implantablepacemakers may also deliver pacing pulses to the right atrium and theright ventricle via the one or more electrodes in the right atrium andthe right ventricle, respectively.

Due to the ability to sense both atrial and ventricular electricalactivity, such dual-chamber implantable pacemakers may be able toprovide atrio-ventricular synchronous pacing. For patients withintermittent AV node conduction, it may be preferable to inhibitventricular pacing and allow an intrinsic ventricular depolarization tooccur for a time, referred to as the AV interval, after an intrinsicatrial depolarization or atrial pace. Such atrio-ventricular synchronouspacing by dual-chamber implantable pacemakers is referred to as the VDDprogramming mode, and may be used for patients with various degrees ofAV block.

Alternatively, dual-chamber implantable pacemakers may provideasynchronous ventricular pacing. Asynchronous ventricular pacing may bepreferable if the patient's heart rate becomes irregular. According toan asynchronous ventricular pacing mode, the dual-chamber implantablepacemaker delivers a ventricular pacing pulse if an intrinsicventricular depolarization is not detected within a “VV interval” thatbegins when a previous intrinsic depolarization was detected, or aprevious ventricular pacing pulse was delivered. Such asynchronousventricular pacing by dual-chamber implantable pacemakers is referred toas the VVI programming mode, or VVIR programming mode if the VV intervalis rate-adaptive (i.e., the implantable pacemaker can sense changes inthe patient's heart rate and alter the VV interval accordingly).

Implantable cardiac leads and the pocket in which pacemakers areimplanted may be associated with complications. To avoid suchcomplications, leadless pacing devices sized to be implanted entirelywithin the heart, e.g., in one chamber, such as the right ventricle, ofthe heart have been proposed. Some proposed leadless pacing devicesinclude a plurality of electrodes that are affixed to, or are a portionof, the housing of the respective leadless pacing device (“LPD”).

In some examples, a LPD described herein is configured to pace in anatrio-synchronous ventricular mode and an asynchronous ventricular mode.As discussed below, the processing module may select the mode with whichthe LPD delivers pacing pulses to a heart of a patient based on adetection of an atrial undersensing event.

Due to the placement of the LPD within a ventricle, the electricalactivity of the right atrium sensed by the electrodes of the LPDimplanted in the ventricle may be relatively low power (e.g., a lowamplitude P-wave), which may cause LPD to not sense an atrial activationevent based on the electrical cardiac signal. This may result in anatrial undersensing event.

In some examples, the processing module controls the LPD to switch fromthe atrio-synchronous ventricular pacing mode to the asynchronousventricular pacing mode in response to detecting an atrial undersensingevent. When switched from the atrio-ventricular synchronous pacing modeto the asynchronous ventricular pacing mode, the LPD terminates deliveryof pacing pulses according to the atrio-ventricular synchronous anddelivers pacing pulses to the patient in accordance with theasynchronous ventricular pacing mode

The processing module may attempt to detect an atrial activation event(e.g., an event that leads to a contraction of an atrium) within anatrial activation event detection window begins at a ventricularactivation event (e.g., an intrinsic ventricular depolarization or adelivery of a pacing pulse to the ventricle). In some examples, anatrial activation event is detection of a far field P-wave in anelectrical cardiac signal sensed by the LPD implanted in the ventricle,a detection of the delivery of an atrial pacing pulse by another device(e.g., another LPD implanted in the atrium), or detection of mechanicalcontraction of the atrium.

In response to determining the atrial activation event was detectedwithin the atrial activation event detection window, the processingmodule may control the LPD to deliver a ventricular pacing pulseaccording to the timing of the atrio-ventricular synchronous pacingmode. For example, in response to further determining a ventricularactivation event was not detected subsequent to the detected atrialactivation event (e.g., within an AV interval), the processing modulemay control the LPD to deliver a ventricular pacing pulse apredetermined interval of time after the detected atrial activationevent.

However, in response to determining that an atrial activation event wasnot detected within the atrial activation event detection window, theprocessing module may increment an undersensing event counter orgenerate an undersensing indication, which may be a flag, value, orother parameter stored by a memory of the LPD or another device. Theprocessing module may, in some examples, deliver a ventricular pacingpulse a time period (e.g., a VV interval plus offset, where the VVinterval may be based on historic VV interval data or a preprogrammed VVinterval) after the prior ventricular activation event.

In response to determining the counter value is greater than or equal toan undersensing event threshold value or after the processing modulegenerates a certain number of undersensing indications (e.g., within aparticular time interval), the processing module may control the LPD toswitch from the atrio-ventricular synchronous pacing mode to anasynchronous ventricular pacing mode. The processing module maydetermine that an undersensing event occurred in response to determiningthe counter value is greater than or equal to a predeterminedundersensing event threshold value or in response to determining numberof undersensing indications is greater than or equal to a predeterminedundersensing event threshold value. The predetermined undersensing eventthreshold value may be, for example, one, while in other examples, thepredetermined threshold value may be more than one, such as two, three,or four or more.

In some examples, the counter may count the number of consecutivecardiac cycles in which an undersensing indication was generated. Inother examples, the counter may count the number of cardiac cycles (“X”)out of a predetermined number of consecutive cardiac cycles (“Y”) inwhich an undersensing indication was generated. This may be referred toas an “X of Y” style counter. In other examples, the counter may countthe number of cardiac cycles, within a predetermined period of time, inwhich an undersensing indication was generated.

In examples in which the predetermined threshold value is more than one,the LPD may not switch to the asynchronous ventricular pacing modeimmediately after detection one instance of atrial undersensing. Thismay permit the heart of the patient to resume intrinsic conduction. Inthis way, the LPD may be configured to determine whether the heartresumes intrinsic conduction before switching to the asynchronousventricular pacing mode. In some situations, the atrio-ventricularsynchronous pacing mode may promote better synchrony of the heart of thepatient. In at least some of these situations, the LPD may be configuredto help promote better synchrony of the heart.

In some examples, an LPD is configured to operate in a sensing withoutpacing mode (e.g., a mode corresponding to the ODO mode of a dualchamber pacemaker with leads). For example, the LPD may operate in thesensing without pacing mode as an initial mode upon implantation of theLPD in a ventricle, prior to delivering any pacing therapy to thepatient. In the sensing without pacing mode, the LPD senses electricalcardiac activity, but does not deliver any pacing therapy to the heartof the patient.

In some examples described herein, an LPD is configured to switch from asensing without pacing mode to an atrio-ventricular synchronous pacingmode in response to detecting, while the LPD is in the sensing withoutpacing mode, ventricular undersensing. When the LPD switches to theatrio-ventricular synchronous pacing mode, the LPD begins deliveringpacing stimulation to the patient.

Ventricular undersensing may occur when the LPD does not detect (e.g.,sense), for a predetermined number of cardiac cycles, a ventricularsense event within a ventricular event detection window V_(ACT) thatbegins at an atrial activation event (which may be an intrinsic event orthe delivery of an atrial pacing pulse). A ventricular sense event mayalso be referred to as a ventricular depolarization event. Eachoccurrence of a ventricular undersensing event may indicate that theventricle did not intrinsically conduct at the expected time followingthe atrial activation event. The ventricle may not intrinsically conductdue to, for example, atrioventricular (AV) block. Thus, the ventricularundersensing may occur due to AV block.

In examples in which the predetermined threshold number of ventricularevents is more than one, the ventricle of the heart of the patient maynot properly depolarize or contract in the cardiac cycle in which theventricular event occurred. As a result, the heart of the patient mayskip a beat. This may be referred to as a “dropping” of a heart beat bythe LPD. By being configured to drop one or more heart beats, the LPDmay be configured to favor the intrinsic conduction of the heart byproviding time for the heart to resume intrinsic conduction or for LPDto sense the intrinsic conduction before the LPD switches to anatrio-ventricular synchronous pacing mode, in which the LPD delivers aventricular pacing pulse that may override the intrinsic conduction ofthe heart. In this way, the LPD may sense the intrinsic activity of theheart for at least one full beat before delivering a ventricular pacingpulse in an atrio-ventricular synchronous pacing mode.

The predetermined threshold number of ventricular events affects thenumber of beats that may be dropped before the LPD switches to anatrio-ventricular synchronous pacing mode. For example, if thepredetermined threshold number of ventricular events is one, then theheart may drop one beat before the LPD switches to the atrio-ventricularsynchronous pacing mode. As another example, if the undersensingthreshold value is two, then the heart may drop two beats before theswitches to the atrio-ventricular synchronous pacing mode.

FIG. 1 is a conceptual diagram illustrating an example leadless pacingdevice (LPD) 10A that is configured to operating in a sensing withoutpacing mode, and deliver atrio-ventricular synchronous pacing andasynchronous ventricular pacing. In some examples, whether LPD 10A isoperating in a sensing without pacing mode or in an atrio-ventricularsynchronous pacing mode is controlled based on the detection ofventricular undersensing, as described in further detail below therespect to FIGS. 6A, 6B, and 7. In addition, in some examples, whetherLPD 10A delivers pacing pulses to a patient in an atrio-ventricularsynchronous pacing mode or an asynchronous ventricular pacing mode iscontrolled based on the detection of an atrial undersensing event, asdescribed in further detail below.

As illustrated in FIG. 1, LPD 10A includes an outer housing 12, fixationtimes 14A-14D (collectively “fixation tines 14”), and electrodes 16A and16B. Outer housing 12 is configured such that, e.g., has a size and formfactor, that allows LPD 10A to be entirely implanted within a chamber ofa heart, such as a right ventricle. As illustrated in FIG. 1, housing 12may have a cylindrical (e.g., pill-shaped) form factor in some examples.Housing 12 may be hermetically sealed to prevent ingress of fluids intothe interior of housing 12.

Fixation tines 14 extend from outer housing 12, and are configured toengage with cardiac tissue to substantially fix a position of housing 12within a chamber of a heart, e.g., at or near an apex of a rightventricle. Fixation tines 14 are configured to anchor housing 12 to thecardiac tissue such that LPD 10A moves along with the cardiac tissueduring cardiac contractions. Fixation tines 14 may be fabricated fromany suitable material, such as a shape memory material (e.g., Nitinol).The number and configuration of fixation tines 14 illustrated in FIG. 1is merely one example, and other numbers and configurations of fixationtines for anchoring an LPD housing to cardiac tissue are contemplated.Additionally, although LPD 10A includes a plurality of fixation tines 14that are configured to anchor LPD 10A to cardiac tissue in a chamber ofa heart, in other examples, LPD 10A may be fixed to cardiac tissue usingother types of fixation mechanisms, such as, but not limited to, barbs,coils, and the like.

LPD 10A is configured to sense electrical activity of a heart, i.e., acardiac electrogram (“EGM”), and deliver pacing pulses to a rightventricle, via electrodes 16A and 16B. Electrodes 16A and 16B may bemechanically connected to housing 12, or may be defined by a portion ofhousing 12 that is electrically conductive. In either case, electrodes16A and 16B are electrically isolated from each other. Electrode 16A maybe referred to as a tip electrode, and fixation tines 14 may beconfigured to anchor LPD 10A to cardiac tissue such that electrode 16Amaintains contact with the cardiac tissue. Electrode 16B may be definedby a conductive portion of housing 12 and, in some examples, may defineat least part of a power source case that houses a power source (e.g., abattery) of LPD 10A. In some examples, a portion of housing 12 may becovered by, or formed from, an insulative material to isolate electrodes16A and 16B from each other and/or to provide a desired size and shapefor one or both of electrodes 16A and 16B.

Outer housing 12 houses electronic components of LPD 10A, e.g., anelectrical sensing module for sensing cardiac electrical activity viaelectrodes 16A and 16B, a sensor, and an electrical stimulation modulefor delivering pacing pulses via electrodes 16A and 16B. Electroniccomponents may include any discrete and/or integrated electronic circuitcomponents that implement analog and/or digital circuits capable ofproducing the functions attributed to an LPD described herein.Additionally, housing 12 may house a memory that includes instructionsthat, when executed by one or more processors housed within housing 12,cause LPD 10A to perform various functions attributed to LPD 10A herein.In some examples, housing 12 may house a communication module thatenables LPD 10A to communicate with other electronic devices, such as amedical device programmer. In some examples, housing 12 may house anantenna for wireless communication. Housing 12 may also house a powersource, such as a battery. The electronic components of LPD 10A aredescribed in further detail below, with respect to FIG. 4.

FIG. 2 is a conceptual diagram illustrating an example leadless pacingsystem 20A that comprises the example LPD 10A from FIG. 1. In theexample of FIG. 2, LPD 10A is implanted within right ventricle 22 ofheart 24 of patient 26. More particularly, LPD 10A is fixed or attachedto the inner wall of the right ventricle 22 proximate to the apex of theright ventricle in the example of FIG. 2 via the fixation tines 14. Inother examples, LPD 10A may be fixed to the inner wall of rightventricle 22 at another location, e.g., on the intraventricular septumor free-wall of right ventricle 22, or may be fixed to the outside ofheart 24, e.g., epicardially, proximate to right ventricle 22. In otherexamples, LPD 10A may be fixed within, on, or near the left-ventricle ofheart 24.

LPD 10A includes a plurality of electrodes that are affixed to, or are aportion of, the housing of LPD 10A, i.e., electrodes 16A and 16B. LPD10A may be configured to sense electrical cardiac signals associatedwith depolarization and repolarization of heart 24, e.g., an EGM, viaelectrodes 16A and 16B. LPD 10A is also configured to deliver cardiacpacing pulses to right ventricle 22 via electrodes 16A and 16B. In someexamples, LPD 10A may deliver the cardiac pacing pulses according to anatrio-ventricular synchronous pacing mode or an asynchronous ventricularpacing mode, depending on whether one or more undersensing events aredetected.

LPD 10A is configured to detect a ventricular activation event in anysuitable way. In some examples, a processing module of LPD 10A isconfigured to detect a ventricular activation event based on ventricularelectrical activity (e.g., an R-wave), which may be indicative of anintrinsic depolarization of right ventricle 22. In addition to, orinstead of, the ventricular electrical activity, the processing moduleis configured to detect a ventricular activation event based on thedelivery of a pacing pulse to right ventricle 22. In yet other examples,the processing module may be configured to detect a ventricularactivation event based on detection of a ventricular contraction, whichmay be detected based on heart sounds (e.g., the S1 heart sounds) sensedby a sensor of LPD 10A, or based on motion of the right ventricle (e.g.,sensed by a motion sensor of LPD 10A or another device).

LPD 10A is configured to detect an atrial activation event in anysuitable way. In some examples, LPD 10A is configured to detect anatrial activation event based on a mechanical contraction of rightatrium 28, based on detection of an atrial depolarization within theelectrical cardiac signal, or based on both the mechanical contractionand the atrial depolarization. LPD 10A may, for example, detect anatrial depolarization by at least detecting a P-wave, which representsatrial depolarization, within the electrical cardiac signal.

In some examples, LPD 10A may, at times, undersense atrial activationevents. For example, the LPD 10A may be unable to reliably detect atrialdepolarizations, e.g., due to the quality of the electrical signalsensed by electrodes 16A, 16B of LPD 10A, or the relatively smallmagnitude of the atrial depolarizations (e.g., small P-wave amplitude)within the sensed electrical cardiac signal. As described in greaterdetail below, in some examples, LPD 10A is configured to switch from anatrio-ventricular synchronous pacing mode to an asynchronous pacing modein response to detecting an atrial undersensing event.

In contrast to LPD 10A, a dual chamber pacemaker that is electricallyconnected to leads that extend into right ventricle 22 and right atrium28 of heart 24, LPD 10A (as well as other LPDs) may sense atrialactivity with electrodes placed within right atrium 28. As a result, theamplitude of the P-wave of an electrical cardiac signal sensed by thedual chamber pacemaker (or other pacemaker with leads in right atrium28) may be larger than the amplitude of the P-wave of an electricalcardiac signal sensed by LPD 10A. An electrical cardiac signal withlarger P-wave amplitudes may result in fewer atrial undersensing events.Thus, a switch from an atrio-ventricular synchronous pacing mode to anasynchronous pacing mode in response to detecting an atrial undersensingevent, as described herein with respect to LPDs, may not be applicableto a dual chamber pacemaker (or other pacemaker with leads in rightatrium 28) or provide improved utility of the dual chamber pacemaker.

As illustrated in FIG. 2, LPD system 20A also includes a medical deviceprogrammer 18, which is configured to program LPD 10A and retrieve datafrom LPD 10A. Programmer 18 may be a handheld computing device, adesktop computing device, a networked computing device, etc. Programmer18 may include a computer-readable storage medium having instructionsthat cause a processing module of programmer 18 to provide the functionsattributed to programmer 18 in the present disclosure. LPD 10A maywirelessly communicate with programmer 18. For example, LPD 10A maytransfer data to programmer 18 and may receive data from programmer 18.Programmer 18 may also wirelessly program and/or wirelessly charge LPD10A.

Data retrieved from LPD 10A using programmer 18 may include electricalcardiac signals stored by LPD 10A that indicate the electrical activityof heart 24, generated undersensing indications and marker channel datathat indicates the occurrence and timing of sensing, diagnosis, andtherapy events associated with LPD 10A, e.g., detection of atrial andventricular depolarizations and delivery of pacing pulses. Datatransferred to LPD 10A using programmer 18 may include, for example,operational programs for LPD 10A that cause LPD 10A to operate asdescribed herein. As examples, data transferred to LPD 10A usingprogrammer 18 may include lengths of any AV intervals, lengths of any VVintervals, atrial contraction detection delay periods, ventricularactivation event detection windows, atrial activation event detectionwindows, and offsets for determining modified atrial activation eventdetection windows, which are each described in further detail below. Itmay also include any threshold values, such as for detecting atrialand/or ventricular contractions, for detecting an undersensing event(e.g., based on a number of undersensing indications), or programmingused by LPD 10A to determine such values based on determined parametersof heart 24, patient 26, or LPD 10A.

FIG. 3A is a conceptual diagram illustrating another example leadlesspacing system 20B that comprises another example LPD 10B configured tooperate in a sensing without pacing mode and an atrio-ventricularsynchronous pacing mode in some examples. In addition, or instead, insome examples, as with LPD 10A, LPD 10B is configured to deliver eitheratrio-ventricular synchronous pacing or asynchronous ventricular pacingbased on a detection of one or more undersensing events. FIG. 3Billustrates LPD 10B in further detail. Leadless pacing system 20B andLPD 10B may be substantially the same as leadless pacing system 20A andLPD 10A described above with respect to FIGS. 1 and 2. Unlike LPD 10A,however, LPD 10B is coupled to a sensing extension 30 that includes anelectrode 32. In some examples, sensing extension 30 may include one ormore additional electrodes having the same polarity as electrode 32.Although not illustrated in FIGS. 3A and 3B, LPD 10B may include anelectrode 16A but may not include electrode 16B, as described above withrespect to LPD 10A and FIG. 1.

Electrode 32 is electrically connected to electronics within a housingof LPD 10B (e.g., an electrical sensing module and a stimulation module)via an electrical conductor 34 of sensing extension 30. In someexamples, electrical conductor 34 is connected to the electronics via anelectrically conductive portion 36A of outer housing 36 of LPD 12B,which may correspond to electrode 16B of LPD 10A (FIG. 1), but may besubstantially completely insulated (e.g., completely electricallyinsulated or nearly completely electrically insulated). Substantiallycompletely electrically insulating conductive portion 36A of housing 36may allow an electrical sensing module of LPD 10B to sense electricalcardiac activity with electrode 32 of sensing extension 30, rather thanconductive portion 36A of housing 36. This may help improve themagnitude of the atrial depolarization present within an electricalcardiac signal sensed via LPD 10B, particularly relative to the examplesin which electrodes 16A, 16B are affixed to, or are a portion of, thehousing of LPD 10A (FIG. 1).

Additionally, as shown in FIGS. 3A and 3B, sensing extension 30 extendsaway from LPD 10B, which enables electrode 32 to be positionedrelatively close to right atrium 28. As a result, an electrical cardiacsignal sensed by LPD 10B via electrodes 16A (FIG. 1) and 32 may includea higher amplitude far-field atrial depolarization signal than anelectrical cardiac signal sensed by LPD 10A via electrodes 16A and 16B(FIG. 1). In this way, sensing extension 30 may facilitate detection ofatrial depolarizations when LPD 10B is implanted in right ventricle 22.In some examples, sensing extension 30 is sized to be entirely implantedwithin right ventricle 22. In other examples, sensing extension 30 issized to extend into right atrium 28.

Despite sensing extension 30, LPD 10B may, at times, be unable to detectdepolarizations of right atrium 28, e.g., due to reduced electricalcardiac signal quality. Reduced electrical cardiac signal quality mayinclude reduced amplitude of the atrial component of the electricalcardiac signal and/or increased noise, which may cause LPD 10B toundersense atrial events when LPD 10B is implanted in right ventricle22. Reduced electrical cardiac signal quality may be caused by, forexample, movement of sensing extension 30 relative to right atrium 28,which may be caused by posture or activity of patient 26, or otherconditions of patient 26, heart 24, and/or LPD 10B. In order to helpprovide responsive pacing therapy, LPD 10B may be configured to deliverpacing pulses to right ventricle 22 in accordance with either theatrio-ventricular synchronous pacing mode or the asynchronousventricular pacing made, based on detection of an atrial undersensingevent.

While the remainder of the disclosure primarily refers to LPD 10A, thedescription of LPD 10A also applies to LPD 10B, as well as other LPDsconfigured to provide both atrio-synchronous ventricular pacing andasynchronous ventricular pacing.

FIG. 4 is a functional block diagram illustrating an exampleconfiguration of an LPD 10A configured to deliver atrio-ventricularsynchronous pacing or asynchronous ventricular pacing based on adetection of an atrial sensing event. LPD 10B of FIGS. 3A and 3B mayhave a similar configuration as LPD 10A. However, electrode 16B of LPD10A may be replaced by electrode 32 of LPD 10B, which may be positioneda greater distance away from electrode 16A and LPD 10B, as describedabove with respect to FIGS. 3A and 3B.

LPD 10A includes processing module 40, memory 42, stimulation module 44,electrical sensing module 46, sensor 48, communication module 50, andpower source 52. Power source 52 may include a battery, e.g., arechargeable or non-rechargeable battery.

Modules included in LPD 10A represent functionality that may be includedin LPD 10A of the present disclosure. Modules of the present disclosuremay include any discrete and/or integrated electronic circuit componentsthat implement analog and/or digital circuits capable of producing thefunctions attributed to the modules herein. For example, the modules mayinclude analog circuits, e.g., amplification circuits, filteringcircuits, and/or other signal conditioning circuits. The modules mayalso include digital circuits, e.g., combinational or sequential logiccircuits, memory devices, and the like. The functions attributed to themodules herein may be embodied as one or more processors, hardware,firmware, software, or any combination thereof. Depiction of differentfeatures as modules is intended to highlight different functionalaspects, and does not necessarily imply that such modules must berealized by separate hardware or software components. Rather,functionality associated with one or more modules may be performed byseparate hardware or software components, or integrated within common orseparate hardware or software components.

Processing module 40 may include any one or more of a microprocessor, acontroller, a digital signal processor (DSP), an application specificintegrated circuit (ASIC), a field-programmable gate array (FPGA), orequivalent discrete or integrated logic circuitry. In some examples,processing module 40 includes multiple components, such as anycombination of one or more microprocessors, one or more controllers, oneor more DSPs, one or more ASICs, or one or more FPGAs, as well as otherdiscrete or integrated logic circuitry. Additionally, althoughillustrated as separate functional components in FIG. 4, some or all ofthe functionality attributed to stimulation module 44, electricalsensing module 46, and communication module 50 may implemented in theone or more combination of one or more microprocessors, one or morecontrollers, one or more DSPs, one or more ASICs, one or more FPGAs,and/or other discrete or integrated logic circuitry that implementsprocessing module 40.

Processing module 40 may communicate with memory 42. Memory 42 mayinclude computer-readable instructions that, when executed by processingmodule 40, cause processing module 40 and any other modules of LPD 10Ato perform the various functions attributed to them herein. Memory 42may include any volatile, non-volatile, magnetic, or electrical media,such as a random access memory (RAM), read-only memory (ROM),non-volatile RAM (NVRAM), electrically-erasable programmable ROM(EEPROM), Flash memory, or any other memory device.

Stimulation module 44 and electrical sensing module 46 are electricallycoupled to electrodes 16A, 16B. Processing module 40 is configured tocontrol stimulation module 44 to generate and deliver pacing pulses toheart 24 (e.g., right ventricle 22 in the example shown in FIG. 2) viaelectrodes 16A, 16B. In addition, processing module 40 is configured tocontrol electrical sensing module 46 to monitor an electrical signalfrom electrodes 16A, 16B in order to monitor electrical activity ofheart 24. Electrical sensing module 46 may include circuits that acquirean electrical signal from electrodes 16A, 16B, as well as circuits tofilter, amplify, and otherwise process the electrical signal. Theelectrical signal includes intrinsic cardiac electrical activity, suchas depolarizations and repolarizations of the ventricles anddepolarizations of the atria, and may be referred to as an electricalcardiac signal or a cardiac electrogram signal. Electrical sensingmodule 46 detects ventricular depolarizations, or ventricular activationevents, within the electrical cardiac signal and detects atrialdepolarizations, or atrial activation events, within the electricalcardiac signal.

In some examples, LPD 10A also includes sensor 48. In some examples,sensor 48 comprises one or more accelerometers. In some examples, sensor48 comprises a plurality of accelerometers, e.g., three accelerometers,each of which is oriented to detect motion in the direction of arespective axis or vector. The axes or vectors may be orthogonal. Inother examples, sensor 48 may comprises one or more different sensorsthat generate a signal as a function of motion, instead of or inaddition to the one or more accelerometers, such as gyros, mercuryswitches, or bonded piezoelectric crystals. In other examples, sensor 48may be a pressure sensor instead of one or more accelerometers.

Communication module 50 may include any suitable hardware (e.g., anantenna), firmware, software, or any combination thereof forcommunicating with another device, such as programmer 18 (FIGS. 2 and 3)or a patient monitor. Under the control of processing module 40,communication module 50 may receive downlink telemetry from and senduplink telemetry to other devices, such as programmer 18 or a patientmonitor, with the aid of an antenna included in communication module 50.

Memory 42 may include data recorded by LPD 10A, e.g., electrical cardiacsignals, heart rates, information regarding detection of atrial sensingevents or undersensing events, undersensing indications, ventricularpacing efficacy, and the like. Under the direction of processing module40, communication module 50 may transfer data recorded by LDP 10A toanother device, such as programmer 18. Memory 42 may also storeprogramming data received by processing module 40 from another device,such as programmer 18, via communication module 50. The programming datastored in memory 42 may include, as examples, lengths of any AVintervals, lengths of any VV intervals, atrial contraction detectiondelay periods, and atrial or ventricular activation event detectionwindows described herein. The programming data stored in memory 42 mayadditionally or alternatively include any threshold values describedhereafter, such as for detecting atrial and/or ventricular contractions,determining whether pacing is efficacious, or determining whetheratrio-ventricular synchronous pacing should be suspended in favor ofasynchronous pacing. The programming data stored in memory 42 mayadditionally or alternatively include data used by processing module 40to determine any values described herein, e.g., based determinedparameters of heart 24, patient 26, or LPD 10A.

FIG. 5 is a timing diagram illustrating normal conduction timing in apatient. The amount of time between an atrial activation event (paced orsensed) and a subsequent ventricular activation event may be generallyreferred to herein as an “A_(ACT)-V_(ACT) interval.” In FIG. 5, theA_(ACT)-V_(ACT) interval has a consistent value of T1, while theintervals between consecutive atrial events (i.e., the A-A interval)consistently have a value of T3. The interval between a ventricularactivation event V_(ACT) and a subsequent atrial activation eventA_(ACT) has a consistent value of T2, and the intervals betweenconsecutive ventricular activation events (i.e., the VV interval) mayconsistently have a value of T4.

FIG. 6A is a timing diagram illustrating an example technique forcontrolling LPD 10A (or another LPD) implanted in right ventricle 22 toswitch from a sensing without pacing mode to an atrio-ventricularsynchronous pacing mode in response to detecting ventricularundersensing. The timing diagram also illustrates an example techniquefor delivering atrio-ventricular synchronous pacing based on detectionof an atrial activation event. An activation of an atrium may be anintrinsic or paced depolarization of the atrium, or a mechanicalcontraction of the atrium. Thus, processing module 40 may identifyactivation of an atrium by determining that electrical sensing module 46detected an intrinsic depolarization of the atrium (e.g., a far fieldP-wave), by determining that another device (e.g., another LPD implantedin the atrium) delivered a pacing pulse to the atrium, or by detectingmechanical contraction of the atrium.

In some examples, in a sensing without pacing mode, processing module 40may detect atrial activation events A_(ACT), e.g., in an electricalcardiac signal sensed by electrical sensing module 46 (FIG. 4), as wellas ventricular sense events V_(S) (e.g., R-waves), e.g., in the sensedelectrical cardiac signal. The ventricular sense event V_(S) mayindicate intrinsic depolarization of the ventricle (e.g., the rightventricle 22) in which LPD 10A is implanted. Processing module 40 maydetermine whether a ventricular sense event V_(S) is detected within aventricular event detection window W_(VACT) that begins at an atrialactivation event A_(S). This may indicate that heart 24 is exhibitingnormal intrinsic conduction.

In some examples, processing module 40 determines the duration of theventricular event detection window W_(VACT) based on the stored datathat indicates the interval between consecutive atrial activation events(i.e., the A-A interval) for a predetermined number of most recentcardiac cycles (e.g., one to 12 beats). This historic A-A interval datamay be stored by memory 42 or a memory of another device. In someexamples, the duration of the ventricular event detection windowW_(VACT) may be one of: the mean A-A interval of the stored A-Aintervals, the median A-A interval of the stored A-A intervals, thegreatest A-A interval of the stored A-A intervals, the shortest A-Ainterval of the stored A-A intervals, or the most recent A-A interval ofthe stored A-A intervals. As another example, the duration of theventricular event detection window W_(VACT) may be a predeterminedpercentage of the stored A-A intervals, such as 30% to about 75% (e.g.,about 50%) of the mean or median A-A interval for the last one to 12beats.

In addition, or instead, the duration of the ventricular event detectionwindow W_(VACT) may be selected by a clinician and may be independent ofthe historic A-A interval data for the patient. For example, in someexamples, the duration of the ventricular event detection windowW_(VACT) is preprogrammed to be a fixed duration between about 300milliseconds (ms) to about 700 ms, such as about 500 ms. Processingmodule 40 may receive the ventricular event detection window W_(VACT)from the clinician via, e.g., a medical device programmer that isconfigured to communicate with LPD 10A.

In yet other examples, processing module 40 determines the duration ofthe ventricular event detection window W_(VACT) based on the stored datathat indicates the interval between consecutive atrial events (i.e., theA-A interval) for a certain number of most recent cardiac cycles and apreprogrammed duration. For example, processing module 40 may determinea first ventricular event detection window W_(VACT) based on the storedA-A interval data, e.g., using the techniques described above, and mayselect the duration of the ventricular event detection window W_(VACT)to be the smaller of the first ventricular event detection windowW_(VACT) and a fixed, programmed duration. In this way, at relativelyslow heart rates, processing module 40 use the fixed, programmedduration as the ventricular event detection window W_(VACT), and athigher heart rates, the duration that is based on the actual cardiacactivity of patient 12 may be used.

In response to determining that, for a particular cardiac cycle, aventricular sense event V_(S) is detected within the ventricular eventdetection window W_(VACT), processing module 40 may continue operatingLPD 10A in the sensing without pacing mode for at least the next cardiaccycle. For example, as shown in FIG. 6A, processing module 40 detectsatrial activation event 60A and a ventricular sense event 62A that isdetected within a time period T₅ of atrial activation event 60A, theduration of time period T₅ being less than the duration of ventricularevent detection window W_(VACT). Atrial activation event 60A andventricular sense event 62A are part of the same cardiac cycle. Thus,processing module 40 maintains the sensing without pacing mode for thenext cardiac cycle. Processing module 40 detects a subsequent normalcardiac cycle in which ventricular sense event 62B is detected byprocessing module 40 within a ventricular event detection windowW_(VACT) of atrial activation event 60B.

During a next cardiac cycle in the example shown in FIG. 6A, however,processing module 40 does not detect a ventricular sense event within aventricular event detection window W_(VACT) of atrial activation event60C. Rather than controlling stimulation module 44 (FIG. 4) to deliver apacing pulse to right ventricle 22 of heart 24 of patient 12 within thesame cardiac cycle as atrial activation event 60C, however, processingmodule 40 may hold off on the delivery of ventricular pacing for atleast one cardiac cycle. This may permit processing module 40 to, forexample, sense the intrinsic conduction of heart 24 and determinewhether heart 24 returns to a normal cardiac rhythm without the aid ofpacing therapy. In this way, processing module 40 may be configured tocontrol pacing therapy to favor intrinsic conduction.

In the example shown in FIG. 6A, in order to determine whether heart 24returns to a normal cardiac rhythm without the aid of pacing therapy,processing module 40 determines whether a ventricular sense event V_(S)is detected within the ventricular event detection window W_(VACT) of anatrial activation event 60D of a subsequent cardiac cycle (immediatelyafter the cardiac cycle in which the ventricular sense event was notdetected within the ventricular event detection window W_(VACT)). Inresponse to detecting a ventricular sense event V_(S) within theventricular event detection window W_(VACT) of an atrial activationevent 60D of a subsequent cardiac cycle, processing module 40 maycontinue to operate LPD 10A in a sensing without pacing mode.

In some examples, processing module 40 may maintain the ventricularevent detection window W_(VACT) for the next cardiac cycle. In otherexamples, however, as shown in FIG. 6A, processing module 40 may shortenthe ventricular event detection window W_(VACT) in the cardiac cyclefollowing the cycle in which the ventricular sense event was notdetected (e.g., in which a beat was dropped). In the example shown inFIG. 6A, the cycle in which the ventricular sense event was not detectedis the cardiac cycle including atrial activation event 60C. Shorteningthe ventricular event detection window W_(VACT) to be a smaller durationventricular event detection window W_(VACTSHORT) in the cardiac cyclefollowing the cycle in which the ventricular sense event was notdetected may help provide more responsive cardiac pacing therapy becauseshortening the ventricular event detection window _(T) may result in amore timely ventricular pacing pulse V_(P) in the subsequent cardiaccycle. In some examples, ventricular event detection windowW_(VACTSHORT) is about 70 ms to about 110 ms, such as about 80 ms in thecardiac cycle including atrial activation event 60D and/or in thecardiac cycle including atrial activation event 60E.

In the example shown in FIG. 6A, however, processing module 40 does notdetect a ventricular sense event V_(S) within the shortened ventricularevent detection window W_(VACTSHORT) that begins at atrial activationevent 60D. In response, processing module 40 switches LPD 10A from thesensing without pacing mode to an atrio-ventricular synchronous pacingmode. According to an example technique for delivering atrio-ventricularsynchronous pacing shown in FIG. 6A, after processing module 40determines that an intrinsic ventricular sense event V_(S) is notdetected within a shortened ventricular event detection windowW_(VACTSHORT) that begins at atrial activation event 60D, processingmodule 40 may control stimulation module 44 to deliver ventricularpacing pulse V_(P) 64A to a ventricle (e.g., right ventricle 22) ofheart 24 a time period T₆ after a detected atrial activation eventA_(ACT). In the example shown in FIG. 6A, time period T₆ has a durationgreater than or equal to the shortened ventricular event detectionwindow W_(VACTSHORT), and may be less than or equal to ventricular eventdetection window W_(VACT). In addition, in the example shown in FIG. 6A,processing module 40 controls stimulation module 44 to deliver theventricular pacing pulse V_(P) 64A about 80 milliseconds after atrialactivation event 60D. Other time intervals may also be used in otherexamples.

Processing module 40 may control the duration of time T₆ between thedetection of atrial activation A_(S) and the delivery of the nextventricular pacing pulse V_(P). In some examples, processing module 40selects the duration of time period T₆ based on the A_(ACT)-V_(S)interval of two or more prior cardiac cycles (e.g., two cardiac cycles,three cardiac cycles, or more), which may be the cardiac cyclesimmediately preceding the first cardiac cycle in which the ventricularsense event was not sensed. For example, the duration of time period T₆may be equal to the average A_(ACT)-V_(S) interval of the two or moreprior cardiac cycles. The average A_(ACT)-V_(S) interval may be betterrepresentative of the current heart rate of patient 26 than, forexample, a preprogrammed atrio-ventricular synchronous pacing interval(A_(ACT)-V_(P)), which affects the pacing interval. In this way,controlling the delivery of ventricular pacing pulse V_(P) relative toan atrial activation event A_(ACT) based on the average A_(ACT)-V_(S)interval of the two or more preceding cardiac cycles may help smooth theheart rate of patient 26, particularly when compared to the delivery ofatrio-ventricular synchronous pacing using a preprogrammedatrio-ventricular synchronous pacing interval (A_(ACT)-V_(P)).

In a next cardiac cycle, processing module 40 detects an atrialactivation event 60E, determines whether a ventricular sense event isdetected within a shortened ventricular event detection windowW_(VACTSHORT) that begins at atrial activation event 60E, and, inresponse to determining the ventricular sense event as not detectedwithin the shortened ventricular event detection window W_(VACTSHORT) orwithin a predetermined AV interval, processing module 40 controlsstimulation module 44 to deliver ventricular pacing pulse V_(P) 64A to aventricle (e.g., right ventricle 22) of heart 24 a time period T₆ afterthe detected atrial activation event A_(ACT) 60E. However, if processingmodule 40 detects the ventricular sense event within the shortenedventricular event detection window W_(VACTSHORT) or the AV interval,then processing module 40 may not control stimulation module 44 todeliver ventricular pacing in that particular cardiac cycle.

In some examples, once processing module 40 switches to theatrio-ventricular synchronous pacing mode, processing module 40 mayapply a different ventricular event detection window W_(VACTSHORT),which may be preprogrammed and associated with the atrio-ventricularsynchronous pacing mode. For example, the ventricular event detectionwindow W_(VACT) applied during the atrio-ventricular synchronous pacingmode may be about 80 ms to about 300 ms such as about 130 ms, althoughother windows may also be used. Thus, in some examples of the techniqueshown in FIG. 6A, the ventricular event detection window W_(VACTSHORT)used by processing module 40 in the cardiac cycle including atrialactivation event 60D may differ from the one used during the cardiaccycle including atrial activation event 60E. As a result, in someexamples, the time periods T₆ may differ in the cardiac cycles includingevents 60D, 60E.

In some examples, the AV interval may be equal to the ventricular eventdetection window W_(VACT) discussed with reference to FIGS. 6A, 6B, and7. In other examples, the AV interval may be less than the ventricularevent detection window W_(VACT). In yet other examples, the AV intervalmay be greater than the ventricular event detection window W_(VACT). Insome examples, the AV interval is preprogrammed, e.g., selected by aclinician. Example AV intervals include, for example, about 80 ms toabout 300 ms range, such as about 130 ms, although other AV intervalsmay be used in other examples.

During operation in the atrio-ventricular synchronous pacing mode,processing module 40 may periodically perform an intrinsic conductioncheck to determine whether heart 24 has returned to normal intrinsicconduction. For example, processing module 40 may control stimulationmodule 44 to withhold ventricular pacing (V_(P)) for at least onecardiac cycle (e.g., one cardiac cycle, two cardiac cycles, or three ormore cardiac cycles) in order to determine whether an intrinsicventricular sense event V_(S) is detected within a ventricular eventdetection window W_(VACT) of an atrial activation event A_(ACT).

In the example shown in FIG. 6A, processing module 40 does not detect aventricular activation event V_(S) within the ventricular eventdetection window W_(VACTSHORT) of atrial activation event A_(ACT) 60F orwithin the ventricular event detection window W_(VACTSHORT) of atrialactivation event A_(ACT) 60G. In response to making this determination,processing module 40 may continue to control stimulation module 44deliver ventricular pacing pulses to right ventricle 22 in theatrio-ventricular synchronous pacing mode. As shown in FIG. 6A, in thismode, processing module 40 may control stimulation module 44 to deliverventricular pacing pulse V_(P) 64C to patient 26 following the atrialactivation event A_(ACT) 60G of a subsequent cardiac cycle.

In examples in which processing module 40 detects an intrinsicventricular sense event V_(S) within the ventricular event detectionwindow W_(VACTSHORT) that begins at a respective atrial activation eventA_(ACT) 60E, processing module 40 may switch back LPD 10A to the sensingwithout pacing mode. In other examples, processing module 40 may switchLPD 10A to the sensing without pacing mode in response to determiningthat, for a predetermined threshold number (e.g., stored by memory 42)of consecutive cardiac cycles, an intrinsic ventricular sense eventV_(S) is detected within the ventricular event detection window A_(ACT)of a respective atrial activation event A_(ACT). The threshold number ofcardiac cycles may be, for example, two, three, four, or more.

In addition, in some examples, processing module 40 may switch LPD 10Ato the sensing without pacing mode in response to determining intrinsicconduction was observed during an intrinsic conduction check. In someexamples, to perform the intrinsic conduction check, processing module40 temporarily places LPD 10A in a sensing without pacing mode for atleast one cardiac cycle (e.g., one cardiac cycle, or two cardiaccycles). In response to determining intrinsic conduction was observedduring at least one cardiac cycle, processing module 40 may control LPD10A to stay in the sensing without pacing mode.

Processing module 40 may perform the intrinsic conduction check at anysuitable interval, which may remain the same or may increase over time.For example, after switching to the atrio-ventricular synchronous pacingmode, processing module 40 may perform the conduction checks atprogressive time intervals that increase over time. As an example, oneminute after switching LPD 10A to the atrio-ventricular synchronouspacing mode, processing module 40 may perform an intrinsic conductiontest. If no intrinsic conduction is tested, then processing module 40may continue operating LPD 10A in the atrio-ventricular synchronouspacing mode, and perform an intrinsic conduction check two minutes afterthe first check. If no intrinsic conduction is detected at that point,then processing module 40 may continue operating LPD 10A in theatrio-ventricular synchronous pacing mode and perform another intrinsicconduction check four minutes after the second check. This may go onwith any suitable progressive time interval.

FIG. 6B is a timing diagram illustrating another example technique forcontrolling LPD 10A (or another LPD) implanted in right ventricle 22 toswitch from a sensing without pacing mode to an atrio-ventricularsynchronous pacing mode in response to detecting ventricularundersensing. As with the technique described with respect to FIG. 6A,processing module 40 controls LPD 10A to switch from a sensing withoutpacing mode, in which no pacing therapy is delivered to heart 24 ofpatient 26, to an atrio-ventricular synchronous pacing mode in responseto determining that, for one or more cardiac cycles, a ventricular senseevent was not detected within a ventricular event detection windowW_(VACT) of an atrial activation event. For example, in FIG. 6B,processing module 40 does not detect a ventricular sense event within aventricular event detection window W_(VACT) of atrial activation event60C. In that cardiac cycle including atrial activation event 60C,processing module 40 may hold off on the delivery of ventricular pacingfor at least one cardiac cycle. Thus, as shown in FIG. 6B, noventricular pacing pulse follows atrial activation event 60C.

As with the technique described with respect to FIG. 6A, processingmodule 40 does not detect a ventricular sense event V_(S) within theshortened ventricular event detection window W_(VACTSHORT) that beginsat atrial activation event 60D and, in response, controls LPD 10A todeliver a pacing pulse V_(P) 64A in accordance with an atrio-ventricularsynchronous pacing mode. In contrast to the technique described withrespect to FIG. 6A, however, in the example shown in FIG. 6B, processingmodule 40 does not control LPD 10A to remain in the atrio-ventricularsynchronous pacing mode after delivering the pacing pulse V_(P) 64A.Instead, processing module 40 controls LPD 10A to revert (e.g., switch)back to the sensing without pacing mode in which LPD 10A does notdeliver any pacing therapy to patient 26.

As shown in FIG. 6B, after LPD 10A delivers pacing pulse V_(P) 64A,processing module 40, while LPD 10A is in a sensing without pacing mode,detects an atrial activation event 60H and determines whether aventricular sense event V_(S) occurs within the shortened ventricularevent detection window W_(VACTSHORT) that begins at atrial activationevent 60H. In the example shown in FIG. 6B, however, processing module40 does not detect a ventricular sense event V_(S) within the shortenedventricular event detection window W_(VACTSHORT). However, becauseprocessing module 40 is controlling LPD 10A in accordance with a sensingwithout pacing mode, processing module 40 does not control stimulationmodule 44 to deliver a ventricular pacing pulse in response todetermining the ventricular sense event V_(S) was not detected withinthe shortened ventricular event detection window W_(VACTSHORT) thatbegins at atrial activation event 60H. In this way, LPD 10A may drop abeat. Instead, processing module 40 may generate an undersensingindication, e.g., by incrementing an undersensing counter, in responseto determining the ventricular sense event V_(S) was not detected withinthe shortened ventricular event detection window W_(VACTSHORT) thatbegins at atrial activation event 60H. Processing module 40 may then, inaccordance with the sensing without pacing mode, detect a subsequentatrial activation event 60I and determine whether a ventricular senseevent V_(S) is detected within the shortened ventricular event detectionwindow W_(VACTSHORT) that begins at atrial activation event 60I.

According to the example technique shown in FIG. 6B, after processingmodule 40 determines that an intrinsic ventricular sense event V_(S) isnot detected within a shortened ventricular event detection windowW_(VACTSHORT) that begins at atrial activation event 60I, processingmodule 40 may control stimulation module 44 to deliver ventricularpacing pulse V_(P) 64D to a ventricle (e.g., right ventricle 22) ofheart 24 a time period T₆ after atrial activation event 60I. Processingmodule 40 may also generate an undersensing indication, e.g., byincrementing an undersensing counter.

In the example shown in FIG. 6B, processing module 40 implements an “Xof Y” style counter, where “X” may be four undersensing indications and“Y” may be four, five, six, or more in some examples. In other examples,“X” may indicate the number of “skipped beats,” e.g., the number ofcardiac cycles in which a ventricular event, whether sensed or paced,was not detected. Thus, in some examples, “X” may be two undersensingindications and “Y” may be three, four, five, six, or more in someexamples, e.g., four.

After processing module 40 increments the counter in response todetermining that the ventricular sense event V_(S) was not detectedwithin the shortened ventricular event detection window W_(VACTSHORT)that begins at atrial activation event 60I, processing module 40 maydetermine that counter indicates the “X” number of undersensingindications out of the “Y” number of cardiac cycles was detected.Accordingly, in response, processing module 40 may switch LPD 10A to theatrio-ventricular synchronous pacing mode indefinitely, e.g., until aconduction check indicates intrinsic conduction is detected or untilanother mode change is made (e.g., until processing module 40 determinesa switch to an asynchronous pacing mode of LPD 10A would be desirable).Unlike in the cardiac cycle including atrial activation event 60D,processing module 40 may not revert LPD 10A back to a sensing withoutpacing mode of operation after LPD 10A delivers the ventricular pacingpulse 64D. Rather, processing module 40 may control stimulation module44 to continue to deliver ventricular pacing pulses in accordance withthe atrio-ventricular synchronous pacing mode.

For example, after switching to LPD 10A to the atrio-ventricularsynchronous pacing mode, processing module 40 may detect an atrialactivation event 60J and determine whether a ventricular sense event isdetected within a shortened ventricular event detection windowW_(VACTSHORT) that begins at atrial activation event 60J. In response todetermining the ventricular sense event as not detected within theshortened ventricular event detection window W_(VACTSHORT) or within apredetermined AV interval, processing module 40 controls stimulationmodule 44 to deliver ventricular pacing pulse V_(P) 64E to a ventricle(e.g., right ventricle 22) of heart 24 a time period T₆ after thedetected atrial activation event A_(ACT) 60J. In contrast, in some casesin which processing module 40 reverts LPD 10A back to the sensingwithout pacing mode, stimulation module 44 does not deliver ventricularpacing pulse V_(P) 64E and heart 24 may skip a beat.

FIG. 7 is a flow diagram of an example technique for operating LPD 10A.In the technique shown in FIG. 7, processing module 40 switches LPD 10Abetween a sensing without pacing mode and an atrio-ventricularsynchronous pacing mode. While the technique shown in FIG. 7, as well asother techniques described herein, are primarily described as beingperformed by processing module 40, in other examples, the techniquesdescribed herein may be performed by another processing module (e.g., aprocessing module of another implanted device or an external device,such as a medical device programmer), alone or in combination withprocessing module 40. In addition, while right atrium 28 and rightventricle 22 are primarily referred to herein, in other examples, thedevices, systems, and techniques described herein may also be used tocontrol pacing therapy delivered to the left ventricle, to senseactivity of the left atrium, or both.

In accordance with the technique shown in FIG. 7, processing module 40identifies an atrial activation event A_(ACT) while operating LPD 10A ina sensing without pacing mode (70). The atrial activation event may be,for example, an intrinsic or paced depolarization of right atrium 28, ora mechanical contraction of right atrium 28. Processing module 40 maydetermine, based on an electrical cardiac signal sensed by electricalsensing module 46, whether a ventricular sense event V_(S) (e.g., anintrinsic ventricular depolarization) is detected within a ventricularevent detection window W_(VACT) that begins at the atrial activationevent A_(ACT) (72). In response to determining the ventricular senseevent V_(S) is detected within the ventricular event detection windowW_(VACT) (“YES” branch of block 72), processing module 40 may reset aventricular event counter, and continue sensing cardiac activity in thesensing without pacing mode (70, 72).

The ventricular event counter may be used to count the number of cardiaccycles in which a ventricular depolarization V_(ACT) was not detectedwithin the ventricular event detection window W_(VACT) that begins at arespective atrial activation event A_(ACT). In the example shown in FIG.7, the ventricular event counter may be used to count the number ofcardiac cycles in which a ventricular depolarization V_(ACT) was notdetected within the ventricular event detection window W_(VACT). Thenumber of cardiac cycles may be consecutive or may be the number ofcardiac cycles (“X”) within a predetermined number of cardiac cycles(“Y”). In the latter example, the counter may be referred to as an “X ofY” style counter, where “X” indicates the number of cardiac cycles inwhich a ventricular depolarization V_(ACT) was not detected and “Y”indicates a predetermined number of consecutive cardiac cycles.

In other examples, the ventricular event counter may be used to countthe number of cardiac cycles within a predetermined period of time inwhich a ventricular depolarization V_(ACT) was not detected within theventricular event detection window W_(VACT). Thus, in other examples ofthe technique shown in FIG. 7, processing module 40 may not reset theventricular event counter (74) in response to detecting that, in onecardiac cycle, the ventricular sense event V_(S) was detected within theventricular event detection window W_(VACT), but, rather, processingmodule 40 may reset the ventricular event counter at the end of apredetermined period of time.

In response to determining the ventricular sense event V_(S) was notdetected within the ventricular event detection window W_(VACT) (“NO”branch of block 72), processing module 40 increments the ventricularevent counter (76). The counter can be implemented by software,hardware, firmware, or any combination thereof. For example, whenprocessing module 40 increments the counter, processing module 40 maygenerate a flag, value or other parameter or indication generated byprocessing module 40 and stored by memory 42 of LPD 10A or a memory ofanother device (e.g., another implanted device or an external medicaldevice programmer). As another example, the counter may be implementedby a register-type circuit and processing module 40 may cause a state ofthe register-type circuit to change in order to increment or otherwisemanage the counter. Counters having other configurations may also beused.

After incrementing the ventricular event counter (76), processing module40 may determine whether the counter value is greater than or equal to aventricular event threshold value (78). The ventricular event thresholdvalue may indicate the number of cardiac cycles for which a ventricularevent may not be detected within a ventricular event detection windowW_(VACT) or a shortened ventricular event detection window W_(VACT)following an atrial activation event before LPD 10A delivers ventricularpacing therapy in an atrio-ventricular synchronous pacing mode. In someexamples, such as the one shown in FIG. 6A, the ventricular eventthreshold value is one. In other examples, the ventricular eventthreshold value may be greater than one, such as two, three, or four ormore.

The ventricular event threshold value may be value determined by aclinician to be indicative of a loss of intrinsic AV conduction, and maybe selected to be low enough to configure LPD 10A to provide aresponsive switch in operation mode, and to provide responsive cardiacrhythm management therapy. The ventricular event threshold value may bestored by memory 42 of LPD 10A or a memory of another device with whichprocessing module 40 may communicate (e.g., a medical device programmer)via communication module 50 (FIG. 4).

In response to determining the counter value is less than theventricular event threshold value (“NO” branch of block 78), processingmodule 40 may continue sensing cardiac activity in the sensing withoutpacing mode (70, 72). On the other hand, in response to determining thecounter value is greater than or equal to the ventricular eventthreshold value (“YES” branch of block 78), processing module 40 mayswitch LPD 10A from the sensing without pacing mode to theatrio-ventricular synchronous pacing mode (80). An example of theatrio-ventricular synchronous pacing mode is described with reference toFIG. 8. A counter value greater than or equal to the ventricular eventthreshold value may indicate the presence of AV block.

In some examples, processing module 40 may reset the counter to zeroeach time a ventricular sense event V_(S) is detected within theventricular event detection window W_(VACT). In other examples,processing module 40 increments the ventricular event counter fornonconsecutive cardiac cycles in which the ventricular depolarization isnot detected within the ventricular event detection window W_(VACT), andresets the counter at other times, e.g., if the ventriculardepolarization is detected within the ventricular event detection windowW_(VACT) for two or more consecutive cardiac cycles. As another example,processing module 40 may manage the ventricular event counter to trackthe number of failures to detect the ventricular sense event V_(S)within the ventricular event detection window W_(VACT) for apredetermined range of time (e.g., within 30 seconds, one minute ormore) or as an “X of Y” style counter. For example, processing module 40may increment the ventricular event counter for each instance, for apredetermined time period, in which a ventricular depolarization is notdetected within the ventricular event detection window W_(VACT), andreset the counter at the end of the time range. As another example,processing module 40 may increment the ventricular event counter foreach instance of a predetermined number of immediately prior cardiaccycles in which a ventricular depolarization is not detected within theventricular event detection window W_(VACT).

In some examples of the technique shown in FIG. 7, processing module 40may generate a ventricular event indication in response to determining aventricular sense event was not detected within the detection windowW_(VACT) and may increment the ventricular event counter by storing theventricular event indication in memory 42 of LPD 10A or a memory ofanother device. Processing module 40 may reset the ventricular eventcounter by deleting the stored ventricular event indications. Processingmodule 40 may also determine whether the ventricular event counter valueis greater than or equal to the threshold value (116) by at leastdetermining whether the number of stored ventricular event indicationsis greater than or equal to the ventricular event threshold value.

The configuration of LPD 10A described with respect to FIGS. 6A, 6B, and7, in which LPD 10A does not deliver ventricular pacing in the cardiaccycle in which a ventricular event was not detected by processing module40, may permit electrical sensing module 46 to sense the intrinsicactivity of heart 24 for at least one full beat before delivering aventricular pacing pulse in an atrio-ventricular synchronous pacingmode. In this way, processing module 40 may take the time to sense theventricular activation event, e.g., to permit heart 24 to resumeintrinsic conduction, before LPD 10A delivers a ventricular pacingpulse, which may or may not correspond to the current heart rhythm ofpatient 26. By being configured to drop one or more heart beats, LPD 10Amay be configured to promote the intrinsic conduction of heart 24 bygiving LPD 10A the opportunity to sense intrinsic conduction of heart 24before stimulation module 44 delivers ventricular pacing. This may helpheart 24 stay synchronized.

FIG. 8 is a flow diagram of an example technique for delivering pacingpulses to a ventricle of heart 24 in accordance with anatrio-ventricular synchronous pacing mode. Processing module 40identifies an atrial activation event (70), determines whether anintrinsic ventricular sense event V_(S) (e.g., an R-wave) is detectedsubsequent to the atrial activation event, e.g., within an AV intervalbeginning when the atrial activation event was detected or within aventricular event detection window W_(VACT).

In response to determining the ventricular sense event was not detectedsubsequent to the atrial activation event (“NO” branch of block 84),processing module 40 may control stimulation module 44 to generate anddeliver a pacing pulse to right ventricle 22 of heart 24 (88). Inresponse to determining the ventricular sense event was detectedsubsequent to the atrial activation event (“YES” branch of block 84),LPD 10A may not deliver a ventricular pacing pulse, but, rather,processing module 40 may continue to monitor the cardiac activity ofpatient 26 (70, 84). For patients with intermittent AV node conduction,it may be preferable to inhibit ventricular pacing by LPD 10A inaccordance with the technique shown in FIG. 8 and allow an intrinsicventricular depolarization to occur for a time, e.g., the AV interval,after an intrinsic atrial depolarization or atrial pace.

In some examples, LPD 10A may undersense atrial activity (e.g.,intrinsic depolarizations or atrial pacing activity) of heart 24, whichmay affect the delivery of ventricular pacing pulses when LPD 10A isoperating in the atrio-ventricular synchronous pacing mode (e.g., asshown in FIG. 8). In accordance with some examples described herein, LPD10A is configured to automatically switch (without user intervention insome cases) from the atrio-ventricular synchronous pacing mode to anasynchronous ventricular pacing mode in response to detecting an atrialundersense event.

FIG. 9 is a timing diagram illustrating an example of asynchronousventricular pacing and is described with reference to FIG. 10, which isa flow diagram of an example technique for delivering asynchronousventricular pacing. In the technique shown in FIG. 10, processing module40 identifies a ventricular activation event V_(ACT) (90), which may bea delivery of a ventricular pacing pulse or an intrinsic depolarizationof right ventricle 22 (e.g., an R-wave in an electrical cardiac signalsensed by sensing module 46). Processing module 40 determines whether anintrinsic depolarization of right ventricle 22 is detected within awithin a VV interval that begins when the ventricular activation wasdetected (e.g., when the previous intrinsic ventricular depolarizationwas detected, or a previous ventricular pacing pulse was delivered)(92).

The VV interval may have any suitable length. In some examples,processing module 40 determines the VV interval based on sensed cardiacactivity of patient 26 and stores in the interval in memory 42. Forexample, processing module 40 may determine the VV interval to be theaverage or median time between consecutive ventricular activation events(e.g., consecutive intrinsic ventricular depolarizations detected byelectrical sensing module 44) for a certain number of cardiac cyclesimmediately preceding the present cardiac cycle. The certain number canbe, for example, two or more, such as six, ten, twelve, twenty, orthirty. In some examples, processing module times the delivery ofventricular pacing pulses delivered in the asynchronous ventricularpacing mode using the VV interval. In this case, the VV interval may beslightly longer than the heart rate of patient 26, which may bedetermined from data from a sensor.

In other examples, processing module 40 may use a preprogrammed VVinterval. This, however, may be lower than the patient's current heartrate, which may cause a relatively abrupt change in the heart rate ofpatient 26, which may not be desired. Controlling the timing ofventricular pacing pulse V_(P) delivered in accordance with theasynchronous ventricular pacing mode based on the VV interval determinedbased on sensed cardiac activity of patient 26 may help smooth the heartrate of patient 26, particularly when compared controlling the timing ofthe pacing pulses based on a preprogrammed rate.

In some examples, processing module 40 determines the VV interval to bethe greater of the VV interval determined based on sensed cardiacactivity of patient 26 or a preprogrammed rate. This may enableprocessing module 40 to provide some minimum pacing rate, which mayfurther help smooth the heart rate of patient 26.

In any of the examples described above, processing module 40 may alsomodify the VV interval based on detected changes in the heart rate ofpatient 26. For example, processing module 40 may increase the VVinterval as heart rate decreases, and decrease the VV interval as theheart rate increases. In this way, processing module 40 may provide rateadaptive asynchronous ventricular pacing.

In response to determining the intrinsic depolarization of rightventricle 22 was detected within the VV interval (“YES” branch of block92), processing module 40 may determine the sensed intrinsicdepolarization of right ventricle 22 was a ventricular activation (90)and determine whether a subsequent intrinsic depolarization of rightventricle 22 is detected within a within a VV interval (92). Forexample, in the timing diagram shown in FIG. 9, after processing module40 identifies ventricular activation event V_(ACT) 98, processing module40 may determine, based on a sensed electrical cardiac signal, that anintrinsic depolarization of right ventricle 22 V_(S) 100 is detectedwithin a VV interval that begins at ventricular activation event V_(ACT)98. Processing module 40 may then determine whether a intrinsicdepolarization of right ventricle 22 is detected within the VV intervalthat begins at intrinsic depolarization of right ventricle 22 V_(S) 100(92)

In response to determining the intrinsic depolarization of rightventricle 22 was not detected within the VV interval (“NO” branch ofblock 92), processing module 40 may control stimulation module 44 todeliver a ventricular pacing pulse to right ventricle 22 (94). Forexample, in the timing diagram shown in FIG. 9, after processing module40 determines, based on a sensed electrical cardiac signal, that anintrinsic depolarization of right ventricle 22 V_(S) was not detectedwithin a VV interval that begins at ventricular sense event V_(s) 100,processing module 40 may control stimulation module 44 to deliver aventricular pacing pulse V_(P) 102 to right ventricle 22. Theventricular pacing pulse may be delivered at the end of the VV intervalthat begins at the prior detected ventricular activation event V_(S)100.

Processing module 40 may then identify ventricular pacing pulse V_(P)102 as a ventricular activation event (90) and determine whether anintrinsic depolarization of right ventricle 22 is detected within awithin a VV interval that begins at ventricular pacing pulse V_(P) 102(92). In the example timing diagram shown in FIG. 9, processing module40 determines, based on a sensed electrical cardiac signal, that anintrinsic depolarization of right ventricle 22 V_(S) was not detectedwithin a VV interval that begins at ventricular pacing pulse V_(P) 102.Thus, processing module 40 may control stimulation module 44 to delivera ventricular pacing pulse V_(P) 104 to right ventricle 22.

FIG. 11 is a flow diagram of an example technique for switching LPD 10A(or another LPD) from an atrio-ventricular synchronous pacing mode toasynchronous ventricular pacing mode in response to detecting an atrialundersensing event. When the switch is performed, processing module 40controls stimulation module 44 to stop delivering pacing pulses to rightventricle 22 in the atrio-ventricular synchronous pacing mode, in whichventricular pacing pulses are timed to an atrial activation event, andcontrols stimulation module 44 to deliver pacing pulses to rightventricle 22 in the asynchronous ventricular pacing mode, in whichventricular pacing pulses are timed relative to a ventricular activationevent.

In accordance with the technique shown in FIG. 11, while processingmodule 40 is controlling stimulation module 44 to deliver ventricularpacing pulses to right ventricle 22 of heart 24 of patient 26 accordingto an atrio-ventricular synchronous pacing mode, processing module 40may detect a ventricular activation event V_(ACT) (91), and determinewhether an atrial activation event A_(ACT) is detected within an atrialevent detection window W_(AACT) that begins at the prior atrialactivation event A_(ACT) (110). In other examples, the atrial eventdetection window W_(AACT) may begin at the prior ventricular activationevent, which can be can be, for example, the delivery of a ventricularpacing pulse (V_(P)) or an intrinsic ventricular depolarization (V_(S))(e.g., an R-wave detected within a sensed electrical cardiac signal). Insome examples, LPD 10A may be configured to detect contraction of rightventricle 22 based on the motion signal, and identify activation of theventricle based on the detected ventricular contraction. Similarly, theatrial activation event can be, for example, an intrinsic atrialdepolarization, an atrial paced event, or a mechanical contraction ofthe atrium sensed by LPD 10A.

The atrial event detection window W_(AACT) may define a listening periodduring which processing module 40 analyzes a sensed electrical cardiacsignal (or another physiological signal) to detect an atrial activationevent. In some examples, the duration of the atrial event detectionwindow W_(AACT) is based on the average or median A-A interval for apredetermined number of past cardiac cycles, such as the past six to 12cardiac cycles. In some examples, the atrial event detection windowW_(AACT) is between about 350 ms and about 1200 ms patients. In yetother examples, the duration of the atrial event detection windowW_(AACT) is based on the average or median VV interval for apredetermined number of past cardiac cycles, such as the past six to 12cardiac cycles.

In response to detecting an atrial activation event A_(ACT) within anatrial event detection window W_(AACT) (“YES” branch of block 110),processing module 40 may reset a counter that is used to track thenumber of cardiac cycles in which an atrial activation event A_(ACT) isnot detected within a respective atrial event detection window W_(AACT)(112), referred to herein as an undersensing counter. In some examples,the undersensing counter may count a number of undersensing indicationsand may be configured in the same way as the ventricular event counter(described with respect to FIG. 7). In some examples, the undersensingcounter may be used to count the number of consecutive cardiac cycles inwhich an atrial activation event A_(ACT) is not detected within arespective atrial event detection window W_(AACT), and processing module40 may reset the counter by returning the counter to zero in someexamples. In other examples, the undersensing counter may be used tocount the number of cardiac cycles within a predetermined period of timeor a predetermined number of cardiac cycles in which an atrialactivation event A_(ACT) is not detected within a respective atrialevent detection window W_(AACT). In some examples in which an “X of Y”type counter is used, processing module 40 may reset the counter byremoving any counts included in the “X” value that correspond toundersensing indications that occurred prior to the “Y” number ofpreceding cardiac cycles. In other examples of the technique shown inFIG. 11, processing module 40 may not reset the undersensing counter(112) in response to detecting that, in one cardiac cycle, the atrialactivation event A_(ACT) is detected within the atrial event detectionwindow, but, rather, processing module 40 may reset the undersensingcounter at the end of a predetermined period of time.

Processing module 40 may identify a next ventricular activation event orthe end of the next atrial activation event detection window (91) andrepeat the technique shown in FIG. 11. In response to determining thatan atrial activation event A_(ACT) is not detected within the nextatrial event detection window W_(AACT) (“NO” branch of block 110),processing module 40 generates an undersensing indication and incrementsthe undersensing counter (114). In some cases, the atrial activationevent may occur, but may be detected by processing module 40 outside ofthe atrial event detection window W_(AACT), after the end of the atrialevent detection window. In other cases, the atrial activation event maynot occur or may not be detected by processing module prior to a nextventricular activation event is detected. In either case, the lack ofdetection of the atrial activation event may be determined to be notwithin an atrial event detection window W_(AACT) and the result ofatrial undersensing by LPD 10A.

Processing module 40 determines whether the undersensing counter valueis greater than or equal to an undersensing threshold value (116). Theundersensing threshold value may indicate the number of cardiac cyclesfor which an atrial activation event may not be detected within anatrial activation event detection window W_(AACT) that begins at animmediately prior detected atrial activation event A_(ACT) beforeprocessing module 40 detects an atrial undersensing event and controlsLPD 10A to switch to the asynchronous ventricular pacing mode. Theundersensing threshold value may be stored by memory 42 of LPD 10A or amemory of another device with which processing module 40 may communicate(e.g., a medical device programmer).

In some examples, the undersensing threshold value is one. In otherexamples, the undersensing threshold value may be greater than one, suchas two, three, or four or more. The undersensing threshold value may beselected to be low enough to configure LPD 10A to responsively switchoperation mode to provide responsive cardiac rhythm management therapy,yet high enough to provide LPD 10A with time to determine whether anatrial activation event will be eventually be sensed by LPD 10A within atime period in which LPD 10A may still deliver effectiveatrio-ventricular synchronous pacing. This may permit heart 24 tomaintain intrinsic conduction because if sensing module 40 of LPD 10Asenses an atrial activation event, then LPD 10A may continue deliveringatrio-ventricular synchronous pacing, which favors synchrony of rightventricle 22 with right atrium 28.

In the atrio-ventricular synchronous pacing mode, processing module 40controls stimulation module 44 to generate and deliver a ventricularpacing pulse to right ventricle 22 (or the left ventricle in otherexamples) a predetermined time period T₆ after a detected atrialactivation event A_(ACT). In this way, the atrial activation eventA_(ACT) may be used to time the delivery of a ventricular pacing pulsein the atrio-ventricular synchronous pacing mode. In some examples, thetime period T₆ is based on stored VV interval data. For example timeperiod T₆ may be selected such that the VV interval for that cardiaccycle is substantially equal (e.g., equal or nearly equal) to average ormedian VV interval for a certain number of immediately preceding cardiaccycles.

In cardiac cycles in which processing module 40 does not detect anatrial activation event A_(ACT) within an atrial event detection windowW_(AACT) (“NO” branch of block 110), processing module 40 may controlthe timing of a ventricular pacing pulse by stimulation module 44 basedon the prior ventricular pacing pulse. As shown in FIG. 12, for example,processing module 40 may control stimulation module 44 to deliver aventricular pacing pulse a time period after the prior ventricularpacing pulse (or other ventricular activation event), the time periodbeing substantially equal to the “VV interval+offset.” The “VVinterval+offset” may be substantially equal to average or median VVinterval for a certain number of immediately preceding cardiac cyclesplus an offset, which adds time to the VV interval. As a result, thepacing pulse may be delivered after time period T₆, but still in atimely manner. In some cases, the additional amount of time is about 50milliseconds to about 250 milliseconds, such as about 100 milliseconds,or a percentage of the VV intervals

By timing the delivery of a ventricular pacing pulse based on a detectedatrial activation event in the atrio-ventricular synchronous mode,processing module 40 may time the ventricular pacing pulse based on thecurrent heart rate of patient 26.

If the undersensing counter value is not greater than or equal to anundersensing threshold value, then processing module 40 may determinethat no atrial undersensing event was detected. Thus, in response todetermining the undersensing counter value is less than the undersensingthreshold value (“NO” branch of block 116), processing module 40 maycontinue to monitor the cardiac activity of patient 26 using thetechnique shown in FIG. 11 until an atrial undersensing event isdetected. In some examples, however, processing module 40 may not detecta ventricular activation event (91), if, for example, stimulation module44 did not deliver a ventricular pacing pulse in the cardiac cycle inwhich the atrial activation event was not detected. Thus, rather thandetecting a ventricular activation event, processing module 40 mayidentify the end of the prior atrial event detection window W_(AACT)(91) and determine whether an atrial activation event is detected withina modified atrial event detection window W_(AACTMOD) that begins at theend of the atrial event detection window W_(AACT). The modified atrialevent detection window W_(AACTMOD) may have a longer duration than theatrial event detection window W_(AACT) in some examples.

In some examples, the atrial event detection window W_(AACT) has aduration that is based on the VV interval of prior detected ventricularactivation events. For example, the atrial event detection window may bethe longest, shortest, or average duration of the last two, three, fouror more (e.g., ten or more) VV intervals for those cardiac cycles inwhich processing module 40 detected atrial activation events, plus anadditional amount of time.

The modified atrial event detection window W_(AACTMOD) may be the atrialevent detection window W_(AACT) plus an additional amount of time toaccount for a change in the heart rate of patient 26. The offset may befrom about 30 ms to about 100 ms, such as about 50 ms or about 100 ms,or may be in a range from about 50 ms to about 150 ms. In otherexamples, the offset may be from about 50 ms to about 150 ms, which maybe in the 10 to 20 beats per minute heart rate range.

Processing module 40 may store the atrial event detection windowW_(AACT), the modified atrial event detection window W_(AACTMOD), theA-A intervals for the latest detected atrial activation events, theoffset, or any combination thereof, in memory 42 of LPD 10A or a memoryof another device.

Processing module 40 may detect an atrial undersensing event in responseto determining the undersensing counter value is greater than or equalto an undersensing threshold value (“YES” branch of block 116). As shownin FIG. 11, in response to determining the undersensing counter value isgreater than or equal to an undersensing threshold value (“YES” branchof block 116), processing module 40 may switch LPD 10A from theatrio-ventricular synchronous pacing mode to an asynchronous ventricularpacing mode (118). Thus, processing module 40 may control the deliveryof ventricular pacing pulses to right ventricle 22 of heart 24 ofpatient 26 using an asynchronous ventricular pacing mode, e.g., such asthat shown and described with respect to FIGS. 9 and 10.

In some examples, processing module 40 generates an indication of theundersensing event and stores the indication in memory 42 or a memory ofanother device. The indication may be, for example, a flag, value, orother parameter. The number and timing of the undersensing events may beused by a clinician at a later time to evaluate the patient condition orthe therapy.

In some examples of the technique shown in FIG. 11, processing module 40may generate an undersensing indication in response to determining anatrial activation event A_(ACT) is not detected within a respectiveatrial event detection window W_(AACT) and may increment theundersensing counter by storing the undersensing indication in memory 42of LPD 10A or a memory of another device. Processing module 40 may resetthe undersensing counter by, for example, deleting the storedundersensing indications, or by deleting the stored undersensingindications that occurred prior to the “Y” number of preceding cardiaccycles when an “X of Y” type counter is used. Processing module 40 mayalso determine whether the undersensing counter value is greater than orequal to the threshold value (116) by at least determining whether thenumber of stored undersensing indications is greater than or equal tothe undersensing threshold value.

FIG. 12 is a timing diagram illustrating example application of thetechnique of FIG. 11 for switching from an atrio-ventricular synchronouspacing mode to an asynchronous ventricular pacing mode in response todetecting an undersensing event. The example shown in FIG. 12, theventricular activation events are shown as ventricular pacing pulsesV_(P), but at least some of the ventricular activation events may beintrinsic ventricular depolarizations in other examples.

As shown in FIG. 12, processing module 40 may control the delivery ofventricular pacing pulses to right ventricle 22 in an atrio-ventricularsynchronous pacing mode. For example, as described with respect to FIG.7, processing module 40 may detect an atrial activation event A_(ACT)120 based on an electrical cardiac signal sensed by electrical sensingmodule 46, based on the detection of the delivery of an atrial pacingpulse, or using another technique. Processing module 40 may determine,e.g., using the technique shown in FIG. 8, whether an intrinsicventricular depolarization is detected within a ventricular sense eventdetection window W_(VACT) or W_(ACTSHORT) (not labeled in FIG. 12). Inthe example shown in FIG. 12, in cardiac cycle 121A, processing module40 determines that an intrinsic ventricular depolarization was notdetected within the ventricular sense event detection window andcontrols stimulation module 44 to deliver a ventricular pacing pulseV_(P) 122 to right ventricle 22 a time period T₆ after the detectedatrial activation event A_(ACT) 120.

In a next cardiac cycle 121B (immediately after cardiac cycle 121A),processing module 40 determines that an atrial activation event A_(ACT)124 is detected within an atrial event detection window W_(AACT) thatbegins at the atrial activation event 120 (110). In accordance with thetechnique shown in FIG. 11, processing module 40 may then reset theundersensing counter (112) and control stimulation module 44 to deliverventricular pacing pulse V_(P) 126 to right ventricle 22 a time periodT₆ after the detected atrial activation event A_(ACT) 124. Similarly, inthe following cardiac cycle 121C, processing module 40 determines thatan atrial activation event A_(ACT) 128 is detected within an atrialevent detection window W_(AACT) of the prior cardiac cycle 121B (110).Processing module 40 may then reset the undersensing counter (112) andcontrol stimulation module 44 to deliver ventricular pacing pulse V_(P)130 to right ventricle 22 a time period T₆ after a detected atrialactivation event A_(ACT) 128.

In the following cardiac cycle 121D, processing module 40 determinesthat an atrial activation event A_(ACT) was not detected within amodified atrial event detection window W_(AACTMOD) that begins at theatrial activation event A_(ACT) 128 (“NO” branch of block 110 in FIG.11). The longer atrial event detection window may be used in thisinstance in order to provide processing module 40 with an atrialactivation event listening period that is sufficiently long. Forexample, in some examples, the longer atrial event detection window maybe greater than the average A-A interval for a predetermined number(e.g., 5, 10, or more) of the immediately preceding cardiac cycles. Asanother example, the longer atrial event detection window may have apredetermined, preprogrammed duration.

In response to determining an atrial activation event A_(ACT) was notdetected within a modified atrial event detection window W_(AACTMOD),processing module 40 may increment the undersensing counter (114) anddetermine whether the undersensing counter value is greater than orequal to the undersensing threshold value (116). In the example shown inFIG. 12, processing module 40 determines in cardiac cycle 121D that theundersensing counter value is not greater than or equal to theundersensing threshold value (“NO” branch of block 116). Thus,processing module 40 may control stimulation module 44 to deliver aventricular pacing pulse V_(P) 132 to right ventricle 22, which is timedto ventricular pacing pulse 130 of the immediately preceding cardiaccycle 121C. In the example shown in FIG. 12, stimulation module 44delivers ventricular pacing pulse V_(P) 132 a time period afterventricular pacing pulse 130, where the time period is substantiallyequal to (e.g., equal to or nearly equal to) a VV interval plus anoffset (VV interval+offset”). The offset may be 30 ms to about 100 ms,such as about 50 ms or about 100 ms. This way, the VV intervals onlyslightly extend due to the atrial undersense.

In the following cardiac cycle 121E, however, processing module 40determines that an atrial activation event A_(ACT) was not detectedwithin a modified atrial event detection window W_(AACTMOD) that beganat the end of the atrial activation event window of the prior cardiaccycle 121D (“NO” branch of block 110 in FIG. 11). Processing module 40may control stimulation module 44 to deliver a ventricular pacing pulseV_(P) 134 to right ventricle 22, which is timed to ventricular pacingpulse 130 of the immediately preceding cardiac cycle 121D.

In addition, in response to determining that an atrial activation eventA_(ACT) was not detected within a modified atrial event detection windowW_(AACTMOD) that began at the end of the atrial activation event windowof the prior cardiac cycle 121D, processing module 40 may increment theundersensing counter (114) and determine whether the undersensingcounter value is greater than or equal to the undersensing thresholdvalue (116). In the example shown in FIG. 12, processing module 40determines in cardiac cycle 121E that the undersensing counter value isgreater than or equal to the undersensing threshold value (“YES” branchof block 116). Thus, processing module 40 determines that anundersensing event is detected and switches LPD 10A to the asynchronousventricular pacing mode. In the example shown in FIG. 12, in the firstcardiac cycle 121F LPD 10A is in the asynchronous pacing mode,stimulation module 44 delivers a ventricular pacing pulse V_(P) 135 toheart 24 a VV interval after pacing pulse 134 of the prior cardiac cycle121E.

As described with respect to FIGS. 9 and 10, in the asynchronousventricular pacing mode, processing module 40 may determine whether anintrinsic depolarization of right ventricle 22 is detected within awithin a VV interval that begins when a ventricular activation wasdetected (e.g., VP 132), and in response to determining the intrinsicdepolarization of right ventricle 22 was not detected within the VVinterval, processing module 40 may control stimulation module 44 todeliver a ventricular pacing pulse V_(P) 134 to right ventricle 22. Inthe example shown in FIG. 12, the time period between subsequent pacingpulses 132, 134 is shown as the VV interval of one or more prior cardiaccycles, plus an offset (e.g., about 30 ms to about 150 ms, such as about50 ms or about 100 ms, though it may vary based on the heart rate). Insome examples, processing module 40 determines the VV interval based onone or more prior cardiac cycles, such as cardiac cycles 121A, 121B. Forexample, the VV interval may be the average interval between ventricularactivation events of the one or more prior cardiac cycles. The VVinterval may indicate the average or median heart rate, such that theoffset results in a pacing rate that may be lower than the priordetected heart rate of patient 26.

On the other hand, if processing module 40 determined the intrinsicdepolarization of right ventricle 22 was detected within the VVinterval, then processing module 40 may not control stimulation module44 to deliver a ventricular pacing pulse V_(P) 134 to right ventricle22.

LPD 10A may intermittently sense atrial activation events, even when inthe asynchronous pacing mode. In some situations, it may be desirablefor LPD 10A to deliver atrio-ventricular synchronous pacing whenpossible in order to help heart 26 maintain synchrony. Thus, in someexamples described herein, after processing module 40 controls LPD 10Ato switch from an atrio-ventricular synchronous pacing mode to anasynchronous ventricular pacing mode, processing module 40 mayperiodically determine whether an atrial activation event is detected.In response to determining an atrial activation is detected, processingmodule 40 may control LPD 10A to revert back to the atrio-ventricularsynchronous pacing mode. An LPD 10A configured in this manner maydeliver rate responsive pacing when the atrial rate is too slow orconsistent atrial undersensing is occurring, while favoring AVsynchronous pacing over asynchronous pacing.

The techniques described in this disclosure, including those attributedto image IMD 16, programmer 24, or various constituent components, maybe implemented, at least in part, in hardware, software, firmware or anycombination thereof. For example, various aspects of the techniques maybe implemented within one or more processors, including one or moremicroprocessors, DSPs, ASICs, FPGAs, or any other equivalent integratedor discrete logic circuitry, as well as any combinations of suchcomponents, embodied in programmers, such as physician or patientprogrammers, stimulators, image processing devices or other devices. Theterm “processor” or “processing circuitry” may generally refer to any ofthe foregoing logic circuitry, alone or in combination with other logiccircuitry, or any other equivalent circuitry.

Such hardware, software, firmware may be implemented within the samedevice or within separate devices to support the various operations andfunctions described in this disclosure. In addition, any of thedescribed units, modules or components may be implemented together orseparately as discrete but interoperable logic devices. Depiction ofdifferent features as modules or units is intended to highlightdifferent functional aspects and does not necessarily imply that suchmodules or units must be realized by separate hardware or softwarecomponents. Rather, functionality associated with one or more modules orunits may be performed by separate hardware or software components, orintegrated within common or separate hardware or software components.

When implemented in software, the functionality ascribed to the systems,devices and techniques described in this disclosure may be embodied asinstructions on a computer-readable medium such as RAM, ROM, NVRAM,EEPROM, FLASH memory, magnetic data storage media, optical data storagemedia, or the like. The instructions may be executed to support one ormore aspects of the functionality described in this disclosure.

Various examples have been described. These and other examples arewithin the scope of the following claims.

What is claimed is:
 1. A method comprising: sensing an electricalcardiac signal of a patient with a leadless pacing device while theleadless pacing device is in a sensing without pacing mode; receiving,by a processing module, the electrical cardiac signal; detecting, by theprocessing module and based on the electrical cardiac signal, an atrialactivation event; determining, by the processing module and based on theelectrical cardiac signal, a ventricular sense event was not detectedwithin a ventricular event detection window that begins at the atrialactivation event; and controlling, by the processing module, theleadless pacing device to switch from the sensing without pacing mode toan atrio-ventricular synchronous pacing mode based on the determinationthat the ventricular sense event was not detected within the ventricularevent detection window, wherein controlling the leadless pacing deviceto switch to the atrio-ventricular synchronous pacing mode comprisescontrolling the leadless pacing device to deliver a pacing pulse to thepatient according to the atrio-ventricular synchronous pacing mode afterdetecting an additional atrial activation event.
 2. The method of claim1, comprising sensing of a first cardiac cycle and a second cardiaccycle immediately following the first cardiac cycle, wherein controllingthe leadless pacing device to deliver the pacing pulse to the patientaccording to the atrio-ventricular synchronous pacing mode comprisescontrolling the leadless pacing device to deliver the pacing pulse tothe patient according to the atrio-ventricular synchronous pacing modein the first cardiac cycle, the method further comprising, in the secondcardiac cycle, controlling, by the processing module, the leadlesspacing device to switch from the atrio-ventricular synchronous pacingmode to the sensing without pacing mode.
 3. The method of claim 2,wherein the electrical cardiac signal comprises a first electricalcardiac signal and the atrial activation event comprises a first atrialactivation event, the method further comprising, after controlling theleadless pacing device to switch from the atrio-ventricular synchronouspacing mode to the sensing without pacing mode: receiving, by theprocessing module, a second electrical cardiac signal of the patientsensed by the leadless pacing device after the first cardiac cycle andwhile the leadless pacing device is in the sensing without pacing mode;detecting, by the processing module and based on the second electricalcardiac signal, a second atrial activation event; determining, by theprocessing module and based on the second electrical cardiac signal, theventricular sense event was not detected within the ventricular eventdetection window that begins at the second atrial activation event; andcontrolling, by the processing module, the leadless pacing device toswitch from the sensing without pacing mode to the atrio-ventricularsynchronous pacing mode based on the determination that the ventricularsense event was not detected within the ventricular event detectionwindow that begins at the second atrial activation event.
 4. The methodof claim 1, wherein controlling the leadless pacing device to switchfrom the sensing without pacing mode to the atrio-ventricularsynchronous pacing mode further comprises: in response to determiningthe ventricular sense event was not detected within the detectionwindow, incrementing a counter; after incrementing the counter,determining, by the processing module, whether a value of the counter isgreater than or equal to a threshold value; and in response todetermining the value of the counter is greater than or equal to athreshold value, controlling, by the processing module, the leadlesspacing device to deliver the pacing pulse according to theatrio-ventricular synchronous pacing.
 5. The method of claim 4, whereinthe threshold value is one.
 6. The method of claim 4, wherein thethreshold value is two or three.
 7. The method of claim 1, whereindetecting the atrial activation event comprises detecting a P wave inthe sensed electrical cardiac signal.
 8. The method of claim 1, whereindetecting the atrial activation event comprises detecting delivery of anatrial pacing pulse by a medical device different than the leadlesspacing device.
 9. The method of claim 1, further comprising storing A-Aintervals for a predetermined number of prior cardiac cycles of thepatient and determining the ventricular event detection window based onthe stored A-A intervals.
 10. The method of claim 9, wherein determiningthe ventricular event detection window comprises determining at leastone of: a mean A-A interval of the stored A-A intervals, a median A-Ainterval of the stored A-A intervals, a greatest A-A interval of thestored A-A intervals, a shortest A-A interval of the stored A-Aintervals, or a most recent A-A interval of the stored A-A intervals.11. The method of claim 10, wherein determining the ventricular eventdetection window comprises determining a predetermined percentage of theat least one of the mean A-A interval, the median A-A interval, thegreatest A-A interval, the shortest A-A interval, or the most recent A-Ainterval.
 12. The method of claim 9, wherein determining the ventricularevent detection window comprises selecting the smaller of a firstventricular event detection window determined based on the stored A-Aintervals for a predetermined number of prior cardiac cycles or asecond, fixed ventricular event detection window stored by the leadlesspacing device.
 13. The method of claim 1, further comprising receiving,by the processing module, data indicating a duration of the ventricularevent detection window from a medical device programmer.
 14. The methodof claim 1, further comprising, after controlling the leadless pacingdevice to deliver the pacing pulse to the patient according to anatrio-ventricular synchronous pacing mode: performing, by the processingmodule, an intrinsic conduction check; in response to detectingintrinsic conduction based on the intrinsic conduction check,controlling, by the processing module, the leadless pacing device toreturn to the sensing without pacing mode; and in response to notdetecting intrinsic conduction based on the intrinsic conduction check,controlling, by the processing module, the leadless pacing device toremain in the atrio-ventricular synchronous pacing mode.
 15. A leadlesspacing system comprising: a leadless pacing device configured to sensean electric cardiac signal and configured to operate in a sensingwithout pacing mode and an atrio-ventricular synchronous pacing mode;and a processing module configured to: receive the electrical cardiacsignal sensed by a leadless pacing device while the leadless pacingdevice is in the sensing without pacing mode, detect, based on theelectrical cardiac signal, an atrial activation event, determine, basedon the electrical cardiac signal, a ventricular sense event was notdetected within a ventricular event detection window that begins at theatrial activation event, and control the leadless pacing device toswitch from the sensing without pacing mode to the atrio-ventricularsynchronous pacing mode based on the determination that the ventricularsense event was not detected within the ventricular event detectionwindow, wherein the processing module is configured to control theleadless pacing device to switch to the atrio-ventricular synchronouspacing mode by at least controlling the leadless pacing device todeliver a pacing pulse to the patient according to the atrio-ventricularsynchronous pacing mode after detecting an additional atrial activationevent.
 16. The leadless pacing system of claim 15, comprising means forsensing of a first cardiac cycle and a second cardiac cycle immediatelyfollowing the first cardiac cycle wherein the processing module isconfigured to control the leadless pacing device to deliver the pacingpulse to the patient according to the atrio-ventricular synchronouspacing mode by at least controlling the leadless pacing device todeliver the pacing pulse to the patient according to theatrio-ventricular synchronous pacing mode in the first cardiac cycle,wherein the processing module is further configured to, in the secondcardiac cycle, control the leadless pacing device to switch from theatrio-ventricular synchronous pacing mode to the sensing without pacingmode.
 17. The leadless pacing system of claim 16, wherein the electricalcardiac signal comprises a first electrical cardiac signal and theatrial activation event comprises a first atrial activation event,wherein the processing module is configured to, after controlling theleadless pacing device to switch from the atrio-ventricular synchronouspacing mode to the sensing without pacing mode: receive a secondelectrical cardiac signal of the patient sensed by the leadless pacingdevice after the first cardiac cycle and while the leadless pacingdevice is in the sensing without pacing mode; detect, based on thesecond electrical cardiac signal, a second atrial activation event;determine, based on the second electrical cardiac signal, theventricular sense event was not detected within the ventricular eventdetection window that begins at the second atrial activation event; andcontrol the leadless pacing device to switch from the sensing withoutpacing mode to the atrio-ventricular synchronous pacing mode based onthe determination that the ventricular sense event was not detectedwithin the ventricular event detection window that begins at the secondatrial activation event.
 18. The leadless pacing system of claim 15,wherein the processing module is further configured to control theleadless pacing device to switch from the sensing without pacing mode tothe atrio-ventricular synchronous pacing mode by at least: in responseto determining the ventricular sense event was not detected within thedetection window, incrementing a counter, after incrementing thecounter, determining, by the processing module, whether a value of thecounter is greater than or equal to a threshold value, and in responseto determining the value of the counter is greater than or equal to athreshold value, controlling, by the processing module, the leadlesspacing device to deliver the pacing pulse according to theatrio-ventricular synchronous pacing.
 19. The leadless pacing system ofclaim 18, wherein the threshold value is one.
 20. The leadless pacingsystem of claim 18, wherein the threshold value is two or three.
 21. Theleadless pacing system of claim 15, wherein the processing module isconfigured to detect the atrial activation event by at least detecting aP wave in the sensed electrical cardiac signal.
 22. The leadless pacingsystem of claim 15, wherein the processing module is configured todetect the atrial activation event by at least detecting delivery of anatrial pacing pulse by a medical device different than the leadlesspacing device.
 23. The leadless pacing system of claim 15, furthercomprising a means for sensing A-A intervals and a memory that storesthe A-A intervals for a predetermined number of prior cardiac cycles ofthe patient, wherein the processing module is further configured todetermine the ventricular event detection window based on the stored A-Aintervals.
 24. The leadless pacing system of claim 23, wherein theprocessing module is configured to determine the ventricular eventdetection window by at least determining at least one of: a mean A-Ainterval of the stored A-A intervals, a median A-A interval of thestored A-A intervals, a greatest A-A interval of the stored A-Aintervals, a shortest A-A interval of the stored A-A intervals, or amost recent A-A interval of the stored A-A intervals.
 25. The leadlesspacing system of claim 24, wherein the processing module is configuredto determine the ventricular event detection window by at leastdetermining a predetermined percentage of the at least one of the meanA-A interval, the median A-A interval, the greatest A-A interval, theshortest A-A interval, or the most recent A-A interval.
 26. The leadlesspacing system of claim 23, wherein the processing module is configuredto determine the ventricular event detection window by at leastselecting the smaller of a first ventricular event detection windowdetermined based on the stored A-A intervals for a predetermined numberof prior cardiac cycles or a second, fixed ventricular event detectionwindow stored by the leadless pacing device.
 27. The leadless pacingsystem of claim 15, wherein the processing module is configured toreceive data indicating a duration of the ventricular event detectionwindow from a medical device programmer.
 28. The leadless pacing systemof claim 15, wherein the processing module is configured to, aftercontrolling the leadless pacing device to deliver the pacing pulse tothe patient according to an atrio-ventricular synchronous pacing mode:control the leadless pacing device to perform an intrinsic conductioncheck, in response to detecting intrinsic conduction based on theintrinsic conduction check, control the leadless pacing device to returnto the sensing without pacing mode, and in response to not detectingintrinsic conduction based on the intrinsic conduction check, controlthe leadless pacing device to remain in the atrio-ventricularsynchronous pacing mode.
 29. The leadless pacing system of claim 15,wherein the processing module is housed in the leadless pacing device.